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Storing multiple arrays in Python | 59,648 | <p>I am writing a program to simulate the actual polling data companies like Gallup or Rasmussen publish daily: www.gallup.com and www.rassmussenreports.com</p>
<p>I'm using a brute force method, where the computer generates some random daily polling data and then calculates three day averages to see if the average of the random data matches pollsters numbers. (Most companies poll numbers are three day averages)</p>
<p>Currently, it works well for one iteration, but my goal is to have it produce the most common simulation that matches the average polling data. I could then change the code of anywhere from 1 to 1000 iterations. </p>
<p>And this is my problem. At the end of the test I have an array in a single variable that looks something like this:</p>
<pre><code>[40.1, 39.4, 56.7, 60.0, 20.0 ..... 19.0]
</code></pre>
<p>The program currently produces one array for each correct simulation. <em>I can store each array in a single variable, but I then have to have a program that could generate 1 to 1000 variables depending on how many iterations I requested!?</em> </p>
<p>How do I avoid this? I know there is an intelligent way of doing this that doesn't require the program to generate variables to store arrays depending on how many simulations I want.</p>
<p>Code testing for McCain:</p>
<pre><code> test = []
while x < 5:
test = round(100*random.random())
mctest.append(test)
x = x +1
mctestavg = (mctest[0] + mctest[1] + mctest[2])/3
#mcavg is real data
if mctestavg == mcavg[2]:
mcwork = mctest
</code></pre>
<p>How do I repeat without creating multiple mcwork vars?</p>
| 1 | 2008-09-12T18:09:02Z | 59,762 | <p>I would strongly consider using <a href="http://numpy.scipy.org" rel="nofollow">NumPy</a> to do this. You get efficient N-dimensional arrays that you can quickly and easily process.</p>
| 1 | 2008-09-12T19:01:00Z | [
"python",
"arrays"
] |
Storing multiple arrays in Python | 59,648 | <p>I am writing a program to simulate the actual polling data companies like Gallup or Rasmussen publish daily: www.gallup.com and www.rassmussenreports.com</p>
<p>I'm using a brute force method, where the computer generates some random daily polling data and then calculates three day averages to see if the average of the random data matches pollsters numbers. (Most companies poll numbers are three day averages)</p>
<p>Currently, it works well for one iteration, but my goal is to have it produce the most common simulation that matches the average polling data. I could then change the code of anywhere from 1 to 1000 iterations. </p>
<p>And this is my problem. At the end of the test I have an array in a single variable that looks something like this:</p>
<pre><code>[40.1, 39.4, 56.7, 60.0, 20.0 ..... 19.0]
</code></pre>
<p>The program currently produces one array for each correct simulation. <em>I can store each array in a single variable, but I then have to have a program that could generate 1 to 1000 variables depending on how many iterations I requested!?</em> </p>
<p>How do I avoid this? I know there is an intelligent way of doing this that doesn't require the program to generate variables to store arrays depending on how many simulations I want.</p>
<p>Code testing for McCain:</p>
<pre><code> test = []
while x < 5:
test = round(100*random.random())
mctest.append(test)
x = x +1
mctestavg = (mctest[0] + mctest[1] + mctest[2])/3
#mcavg is real data
if mctestavg == mcavg[2]:
mcwork = mctest
</code></pre>
<p>How do I repeat without creating multiple mcwork vars?</p>
| 1 | 2008-09-12T18:09:02Z | 59,778 | <p>Would something like this work?</p>
<pre><code>from random import randint
mcworks = []
for n in xrange(NUM_ITERATIONS):
mctest = [randint(0, 100) for i in xrange(5)]
if sum(mctest[:3])/3 == mcavg[2]:
mcworks.append(mctest) # mcavg is real data
</code></pre>
<p>In the end, you are left with a list of valid <code>mctest</code> lists.</p>
<p>What I changed:</p>
<ul>
<li>Used a <a href="http://docs.python.org/tut/node7.html#SECTION007140000000000000000" rel="nofollow">list comprehension</a> to build the data instead of a for loop</li>
<li>Used <code>random.randint</code> to get random integers</li>
<li>Used <a href="http://docs.python.org/tut/node5.html" rel="nofollow">slices</a> and <code>sum</code> to calculate the average of the first three items</li>
<li>(To answer your actual question :-) ) Put the results in a list <code>mcworks</code>, instead of creating a new variable for every iteration</li>
</ul>
| 2 | 2008-09-12T19:08:38Z | [
"python",
"arrays"
] |
How to retrieve an element from a set without removing it? | 59,825 | <p>Suppose the following:</p>
<pre><code>>>>s = set([1, 2, 3])
</code></pre>
<p>How do I get a value (any value) out of s without doing s.pop()? I want to leave the item in the set until I am sure I can remove it - something I can only be sure of after an asynchronous call to another host.</p>
<p>Quick and dirty:</p>
<pre><code>>>>elem = s.pop()
>>>s.add(elem)
</code></pre>
<p>But do you know of a better way? Ideally in constant time.</p>
| 154 | 2008-09-12T19:58:33Z | 59,841 | <p>Two options that don't require copying the whole set:</p>
<pre><code>for e in s:
break
# e is now an element from s
</code></pre>
<p>Or...</p>
<pre><code>e = next(iter(s))
</code></pre>
<p>But in general, sets don't support indexing or slicing.</p>
| 197 | 2008-09-12T20:08:10Z | [
"python",
"set"
] |
How to retrieve an element from a set without removing it? | 59,825 | <p>Suppose the following:</p>
<pre><code>>>>s = set([1, 2, 3])
</code></pre>
<p>How do I get a value (any value) out of s without doing s.pop()? I want to leave the item in the set until I am sure I can remove it - something I can only be sure of after an asynchronous call to another host.</p>
<p>Quick and dirty:</p>
<pre><code>>>>elem = s.pop()
>>>s.add(elem)
</code></pre>
<p>But do you know of a better way? Ideally in constant time.</p>
| 154 | 2008-09-12T19:58:33Z | 59,928 | <p>Another option is to use a dictionary with values you don't care about. E.g.,</p>
<pre><code>
poor_man_set = {}
poor_man_set[1] = None
poor_man_set[2] = None
poor_man_set[3] = None
...
</code></pre>
<p>You can treat the keys as a set except that they're just an array:</p>
<pre><code>
keys = poor_man_set.keys()
print "Some key = %s" % keys[0]
</code></pre>
<p>A side effect of this choice is that your code will be backwards compatible with older, pre-<code>set</code> versions of Python. It's maybe not the best answer but it's another option.</p>
<p>Edit: You can even do something like this to hide the fact that you used a dict instead of an array or set:</p>
<pre><code>
poor_man_set = {}
poor_man_set[1] = None
poor_man_set[2] = None
poor_man_set[3] = None
poor_man_set = poor_man_set.keys()
</code></pre>
| 1 | 2008-09-12T20:52:06Z | [
"python",
"set"
] |
How to retrieve an element from a set without removing it? | 59,825 | <p>Suppose the following:</p>
<pre><code>>>>s = set([1, 2, 3])
</code></pre>
<p>How do I get a value (any value) out of s without doing s.pop()? I want to leave the item in the set until I am sure I can remove it - something I can only be sure of after an asynchronous call to another host.</p>
<p>Quick and dirty:</p>
<pre><code>>>>elem = s.pop()
>>>s.add(elem)
</code></pre>
<p>But do you know of a better way? Ideally in constant time.</p>
| 154 | 2008-09-12T19:58:33Z | 60,027 | <p>Since you want a random element, this will also work:</p>
<pre><code>>>> import random
>>> s = set([1,2,3])
>>> random.sample(s, 1)
[2]
</code></pre>
<p>The documentation doesn't seem to mention performance of <code>random.sample</code>. From a really quick empirical test with a huge list and a huge set, it seems to be constant time for a list but not for the set. Also, iteration over a set isn't random; the order is undefined but predictable:</p>
<pre><code>>>> list(set(range(10))) == range(10)
True
</code></pre>
<p>If randomness is important and you need a bunch of elements in constant time (large sets), I'd use <code>random.sample</code> and convert to a list first:</p>
<pre><code>>>> lst = list(s) # once, O(len(s))?
...
>>> e = random.sample(lst, 1)[0] # constant time
</code></pre>
| 15 | 2008-09-12T21:43:27Z | [
"python",
"set"
] |
How to retrieve an element from a set without removing it? | 59,825 | <p>Suppose the following:</p>
<pre><code>>>>s = set([1, 2, 3])
</code></pre>
<p>How do I get a value (any value) out of s without doing s.pop()? I want to leave the item in the set until I am sure I can remove it - something I can only be sure of after an asynchronous call to another host.</p>
<p>Quick and dirty:</p>
<pre><code>>>>elem = s.pop()
>>>s.add(elem)
</code></pre>
<p>But do you know of a better way? Ideally in constant time.</p>
| 154 | 2008-09-12T19:58:33Z | 60,233 | <p>Least code would be:</p>
<pre><code>>>> s = set([1, 2, 3])
>>> list(s)[0]
1
</code></pre>
<p>Obviously this would create a new list which contains each member of the set, so not great if your set is very large.</p>
| 37 | 2008-09-13T01:07:38Z | [
"python",
"set"
] |
How to retrieve an element from a set without removing it? | 59,825 | <p>Suppose the following:</p>
<pre><code>>>>s = set([1, 2, 3])
</code></pre>
<p>How do I get a value (any value) out of s without doing s.pop()? I want to leave the item in the set until I am sure I can remove it - something I can only be sure of after an asynchronous call to another host.</p>
<p>Quick and dirty:</p>
<pre><code>>>>elem = s.pop()
>>>s.add(elem)
</code></pre>
<p>But do you know of a better way? Ideally in constant time.</p>
| 154 | 2008-09-12T19:58:33Z | 61,140 | <p>I use a utility function I wrote. Its name is somewhat misleading because it kind of implies it might be a random item or something like that.</p>
<pre><code>def anyitem(iterable):
try:
return iter(iterable).next()
except StopIteration:
return None
</code></pre>
| 4 | 2008-09-14T04:57:51Z | [
"python",
"set"
] |
How to retrieve an element from a set without removing it? | 59,825 | <p>Suppose the following:</p>
<pre><code>>>>s = set([1, 2, 3])
</code></pre>
<p>How do I get a value (any value) out of s without doing s.pop()? I want to leave the item in the set until I am sure I can remove it - something I can only be sure of after an asynchronous call to another host.</p>
<p>Quick and dirty:</p>
<pre><code>>>>elem = s.pop()
>>>s.add(elem)
</code></pre>
<p>But do you know of a better way? Ideally in constant time.</p>
| 154 | 2008-09-12T19:58:33Z | 1,612,654 | <p>To provide some timing figures behind the different approaches, consider the following code.
<em>The get() is my custom addition to Python's setobject.c, being just a pop() without removing the element.</em></p>
<pre><code>from timeit import *
stats = ["for i in xrange(1000): iter(s).next() ",
"for i in xrange(1000): \n\tfor x in s: \n\t\tbreak",
"for i in xrange(1000): s.add(s.pop()) ",
"for i in xrange(1000): s.get() "]
for stat in stats:
t = Timer(stat, setup="s=set(range(100))")
try:
print "Time for %s:\t %f"%(stat, t.timeit(number=1000))
except:
t.print_exc()
</code></pre>
<p>The output is:</p>
<pre><code>$ ./test_get.py
Time for for i in xrange(1000): iter(s).next() : 0.433080
Time for for i in xrange(1000):
for x in s:
break: 0.148695
Time for for i in xrange(1000): s.add(s.pop()) : 0.317418
Time for for i in xrange(1000): s.get() : 0.146673
</code></pre>
<p>This means that the <strong><em>for/break</em></strong> solution is the fastest (sometimes faster than the custom get() solution).</p>
| 28 | 2009-10-23T10:47:49Z | [
"python",
"set"
] |
How to retrieve an element from a set without removing it? | 59,825 | <p>Suppose the following:</p>
<pre><code>>>>s = set([1, 2, 3])
</code></pre>
<p>How do I get a value (any value) out of s without doing s.pop()? I want to leave the item in the set until I am sure I can remove it - something I can only be sure of after an asynchronous call to another host.</p>
<p>Quick and dirty:</p>
<pre><code>>>>elem = s.pop()
>>>s.add(elem)
</code></pre>
<p>But do you know of a better way? Ideally in constant time.</p>
| 154 | 2008-09-12T19:58:33Z | 34,973,737 | <p>Following @wr. post, I get similar results (for Python3.5)</p>
<pre><code>from timeit import *
stats = ["for i in range(1000): next(iter(s))",
"for i in range(1000): \n\tfor x in s: \n\t\tbreak",
"for i in range(1000): s.add(s.pop())"]
for stat in stats:
t = Timer(stat, setup="s=set(range(100000))")
try:
print("Time for %s:\t %f"%(stat, t.timeit(number=1000)))
except:
t.print_exc()
</code></pre>
<p>Output:</p>
<pre><code>Time for for i in range(1000): next(iter(s)): 0.205888
Time for for i in range(1000):
for x in s:
break: 0.083397
Time for for i in range(1000): s.add(s.pop()): 0.226570
</code></pre>
<p>However, when changing the underlying set (e.g. call to <code>remove()</code>) things go badly for the iterable examples (<code>for</code>, <code>iter</code>):</p>
<pre><code>from timeit import *
stats = ["while s:\n\ta = next(iter(s))\n\ts.remove(a)",
"while s:\n\tfor x in s: break\n\ts.remove(x)",
"while s:\n\tx=s.pop()\n\ts.add(x)\n\ts.remove(x)"]
for stat in stats:
t = Timer(stat, setup="s=set(range(100000))")
try:
print("Time for %s:\t %f"%(stat, t.timeit(number=1000)))
except:
t.print_exc()
</code></pre>
<p>Results in:</p>
<pre><code>Time for while s:
a = next(iter(s))
s.remove(a): 2.938494
Time for while s:
for x in s: break
s.remove(x): 2.728367
Time for while s:
x=s.pop()
s.add(x)
s.remove(x): 0.030272
</code></pre>
| 2 | 2016-01-24T08:38:38Z | [
"python",
"set"
] |
How to retrieve an element from a set without removing it? | 59,825 | <p>Suppose the following:</p>
<pre><code>>>>s = set([1, 2, 3])
</code></pre>
<p>How do I get a value (any value) out of s without doing s.pop()? I want to leave the item in the set until I am sure I can remove it - something I can only be sure of after an asynchronous call to another host.</p>
<p>Quick and dirty:</p>
<pre><code>>>>elem = s.pop()
>>>s.add(elem)
</code></pre>
<p>But do you know of a better way? Ideally in constant time.</p>
| 154 | 2008-09-12T19:58:33Z | 40,054,478 | <h2>tl;dr</h2>
<p><code>for first_item in muh_set: break</code> remains the optimal approach in Python 3.x. <sup><em>Curse you, Guido.</em></sup></p>
<h2>y u do this</h2>
<p>Welcome to yet another set of Python 3.x timings, extrapolated from <a href="https://stackoverflow.com/users/101430/wr">wr.</a>'s excellent <a href="https://stackoverflow.com/a/1612654/2809027">Python 2.x-specific response</a>. Unlike <a href="https://stackoverflow.com/users/2750492/achampion">AChampion</a>'s equally helpful <a href="https://stackoverflow.com/a/34973737/2809027">Python 3.x-specific response</a>, the timings below <em>also</em> time outlier solutions suggested above â including:</p>
<ul>
<li><code>list(s)[0]</code>, <a href="https://stackoverflow.com/users/2168/john">John</a>'s novel <a href="https://stackoverflow.com/a/60233/2809027">sequence-based solution</a>.</li>
<li><code>random.sample(s, 1)</code>, <a href="https://stackoverflow.com/users/3002/df">dF.</a>'s eclectic <a href="https://stackoverflow.com/a/60027/2809027">RNG-based solution</a>.</li>
</ul>
<h2>Code Snippets for Great Joy</h2>
<p>Turn on, tune in, time it:</p>
<pre><code>from timeit import Timer
stats = [
"for i in range(1000): \n\tfor x in s: \n\t\tbreak",
"for i in range(1000): next(iter(s))",
"for i in range(1000): s.add(s.pop())",
"for i in range(1000): list(s)[0]",
"for i in range(1000): random.sample(s, 1)",
]
for stat in stats:
t = Timer(stat, setup="import random\ns=set(range(100))")
try:
print("Time for %s:\t %f"%(stat, t.timeit(number=1000)))
except:
t.print_exc()
</code></pre>
<h2>Quickly Obsoleted Timeless Timings</h2>
<p><strong>Behold!</strong> Ordered by fastest to slowest snippets:</p>
<pre><code>$ ./test_get.py
Time for for i in range(1000):
for x in s:
break: 0.249871
Time for for i in range(1000): next(iter(s)): 0.526266
Time for for i in range(1000): s.add(s.pop()): 0.658832
Time for for i in range(1000): list(s)[0]: 4.117106
Time for for i in range(1000): random.sample(s, 1): 21.851104
</code></pre>
<h2>Faceplants for the Whole Family</h2>
<p>Unsurprisingly, <strong>manual iteration remains at least twice as fast</strong> as the next fastest solution. Although the gap has decreased from the Bad Old Python 2.x days (in which manual iteration was at least four times as fast), it disappoints the <a href="https://www.python.org/dev/peps/pep-0020/" rel="nofollow">PEP 20</a> zealot in me that the most verbose solution is the best. At least converting a set into a list just to extract the first element of the set is as horrible as expected. <em>Thank Guido, may his light continue to guide us.</em></p>
<p>Surprisingly, the <strong>RNG-based solution is absolutely horrible.</strong> List conversion is bad, but <code>random</code> <em>really</em> takes the awful-sauce cake. So much for the <a href="http://catb.org/esr/jargon/html/R/Random-Number-God.html" rel="nofollow">Random Number God</a>.</p>
<p>I just wish the amorphous They would PEP up a <code>set.get_first()</code> method for us already. If you're reading this, They: "Please. Do something."</p>
| 1 | 2016-10-15T03:00:43Z | [
"python",
"set"
] |
Automate firefox with python? | 60,152 | <p>Been scouring the net for something like firewatir but for python. I'm trying to automate firefox on linux. Any suggestions?</p>
| 9 | 2008-09-12T23:28:16Z | 60,218 | <p>The languages of choice of Firefox is Javascript. Unless you have a specific requirement that requires Python, I would advice you to use that.</p>
| 0 | 2008-09-13T00:46:40Z | [
"python",
"linux",
"firefox",
"ubuntu",
"automation"
] |
Automate firefox with python? | 60,152 | <p>Been scouring the net for something like firewatir but for python. I'm trying to automate firefox on linux. Any suggestions?</p>
| 9 | 2008-09-12T23:28:16Z | 60,219 | <p>You could try <a href="http://selenium.openqa.org/">selenium</a>.</p>
| 5 | 2008-09-13T00:48:35Z | [
"python",
"linux",
"firefox",
"ubuntu",
"automation"
] |
Automate firefox with python? | 60,152 | <p>Been scouring the net for something like firewatir but for python. I'm trying to automate firefox on linux. Any suggestions?</p>
| 9 | 2008-09-12T23:28:16Z | 60,548 | <p>See if <a href="http://twill.idyll.org/" rel="nofollow">twill</a> can help you. It can be used as a command line tool or as a python library.</p>
| 1 | 2008-09-13T13:51:57Z | [
"python",
"linux",
"firefox",
"ubuntu",
"automation"
] |
Automate firefox with python? | 60,152 | <p>Been scouring the net for something like firewatir but for python. I'm trying to automate firefox on linux. Any suggestions?</p>
| 9 | 2008-09-12T23:28:16Z | 60,552 | <p>Install <a href="http://hyperstruct.net/projects/mozlab" rel="nofollow">Mozlab</a> in Firefox and enable the telnet server, then open a socket.</p>
| 0 | 2008-09-13T14:06:18Z | [
"python",
"linux",
"firefox",
"ubuntu",
"automation"
] |
Automate firefox with python? | 60,152 | <p>Been scouring the net for something like firewatir but for python. I'm trying to automate firefox on linux. Any suggestions?</p>
| 9 | 2008-09-12T23:28:16Z | 60,630 | <p>I use <a href="http://selenium-rc.openqa.org/python.html" rel="nofollow">Selenium RC</a>. All my tests are written in Python and are run with test suite.</p>
<p>One minor thing is that You either have to start selenium manually and point Your tests to it or start selenium from test suite which requires little bit of coding. But it's doable.</p>
<p>Generally I'm very pleased with this solution.</p>
| 2 | 2008-09-13T15:48:18Z | [
"python",
"linux",
"firefox",
"ubuntu",
"automation"
] |
Automate firefox with python? | 60,152 | <p>Been scouring the net for something like firewatir but for python. I'm trying to automate firefox on linux. Any suggestions?</p>
| 9 | 2008-09-12T23:28:16Z | 69,867 | <p>The <a href="https://developer.mozilla.org/en/PyXPCOM" rel="nofollow">PyXPCOM extension</a> is one possibility.</p>
<p>But looking at what firewatir provides, I have to 2nd the suggestion for twill. It's based on <a href="http://wwwsearch.sourceforge.net/mechanize/" rel="nofollow">mechanize</a>, which might also be useful in this context.</p>
| 3 | 2008-09-16T06:41:51Z | [
"python",
"linux",
"firefox",
"ubuntu",
"automation"
] |
Automate firefox with python? | 60,152 | <p>Been scouring the net for something like firewatir but for python. I'm trying to automate firefox on linux. Any suggestions?</p>
| 9 | 2008-09-12T23:28:16Z | 2,223,726 | <p>Many command line tools don't have a javascript interpreter so do not support web 2.0 functionality. <a href="http://code.google.com/p/juicedpyshell" rel="nofollow">juicedpyshell</a> is based on PyXPCOMext's PyShell example. It gives you a python shell window "inside" the browser, and simplifies access to both the DOM of what you are browsing to and also the shell window itself (so you can add GUI elements as part of your automation script). But its a new project so probably not as full featured as some of the above.</p>
| 0 | 2010-02-08T18:01:59Z | [
"python",
"linux",
"firefox",
"ubuntu",
"automation"
] |
Automate firefox with python? | 60,152 | <p>Been scouring the net for something like firewatir but for python. I'm trying to automate firefox on linux. Any suggestions?</p>
| 9 | 2008-09-12T23:28:16Z | 7,610,441 | <p>I would suggest you to use Selenium instead of Mechanize/Twill because Mechanize would fail while handling Javascript.</p>
| 1 | 2011-09-30T12:35:43Z | [
"python",
"linux",
"firefox",
"ubuntu",
"automation"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 60,211 | <p>You could use a dictionary:</p>
<pre><code>def f(x):
return {
'a': 1,
'b': 2,
}[x]
</code></pre>
| 616 | 2008-09-13T00:38:24Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 60,215 | <p>I've always liked doing it this way</p>
<pre><code>result = {
'a': lambda x: x * 5,
'b': lambda x: x + 7,
'c': lambda x: x - 2
}[value](x)
</code></pre>
<p><a href="http://blog.simonwillison.net/post/57956755106/switch">From here</a></p>
| 156 | 2008-09-13T00:41:46Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 60,216 | <p>If you are really just returning a predetermined, fixed value, you could create a dictionary with all possible input indexes as the keys, along with their corresponding values. Also, you might not really want a function to do this - unless you're computing the return value somehow.</p>
<p>Oh, and if you feel like doing something switch-like, see <a href="https://web.archive.org/web/20120216013404/http://www.mustap.com/pythonzone_post_224_python-switch-statement?" rel="nofollow">here</a>.</p>
| 1 | 2008-09-13T00:43:49Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 60,236 | <p>In addition to the dictionary methods (which I really like, BTW), you can also use if-elif-else to obtain the switch/case/default functionality:</p>
<pre><code>if x == 'a':
# Do the thing
elif x == 'b':
# Do the other thing
if x in 'bc':
# Fall-through by not using elif, but now the default case includes case 'a'!
elif x in 'xyz':
# Do yet another thing
else:
# Do the default
</code></pre>
<p>This of course is not identical to switch/case - you cannot have fall-through as easily as leaving off the break; statement, but you can have a more complicated test. Its formatting is nicer than a series of nested ifs, even though functionally that's what it is closer to.</p>
| 127 | 2008-09-13T01:10:58Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 60,243 | <p>There's a pattern that I learned from Twisted Python code.</p>
<pre><code>class SMTP:
def lookupMethod(self, command):
return getattr(self, 'do_' + command.upper(), None)
def do_HELO(self, rest):
return 'Howdy ' + rest
def do_QUIT(self, rest):
return 'Bye'
SMTP().lookupMethod('HELO')('foo.bar.com') # => 'Howdy foo.bar.com'
SMTP().lookupMethod('QUIT')('') # => 'Bye'
</code></pre>
<p>You can use it any time you need to dispatch on a token and execute extended piece of code. In a state machine you would have <code> state_ </code> methods, and dispatch on <code>self.state</code>. This switch can be cleanly extended by inheriting from base class and defining your own <code> do_ </code> methods. Often times you won't even have <code> do_ </code> methods in the base class.</p>
<p><em>Edit: how exactly is that used</em></p>
<p>In case of SMTP you will receive <code>HELO</code> from the wire. The relevant code (from <code>twisted/mail/smtp.py</code>, modified for our case) looks like this</p>
<pre><code>class SMTP:
# ...
def do_UNKNOWN(self, rest):
raise NotImplementedError, 'received unknown command'
def state_COMMAND(self, line):
line = line.strip()
parts = line.split(None, 1)
if parts:
method = self.lookupMethod(parts[0]) or self.do_UNKNOWN
if len(parts) == 2:
return method(parts[1])
else:
return method('')
else:
raise SyntaxError, 'bad syntax'
SMTP().state_COMMAND(' HELO foo.bar.com ') # => Howdy foo.bar.com
</code></pre>
<p>You'll receive <code>' HELO foo.bar.com '</code> (or you might get <code>'QUIT'</code> or <code>'RCPT TO: foo'</code>). This is tokenized into <code>parts</code> as <code>['HELO', 'foo.bar.com']</code>. The actual method lookup name is taken from <code>parts[0]</code>.</p>
<p>(The original method is also called <code>state_COMMAND</code>, because it uses the same pattern to implement a state machine, i.e. <code>getattr(self, 'state_' + self.mode)</code>)</p>
| 32 | 2008-09-13T01:26:26Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 64,756 | <p>The switch statement is just syntactical sugar which is probably why Python doesn't have it. You can use if else statements for this functionality easily.</p>
<p>Like Matthew Schinckel said, you can use if and elif and else.</p>
<p>It is also a simple matter to have "fall-through" capabilities like most switch statements. All you have to do is not use elif.</p>
<pre><code>if x == 1:
# 1
if x == 2:
# fall-through
elif x == 3:
# not fall-through
</code></pre>
| -1 | 2008-09-15T17:06:26Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 67,450 | <p>I would just use if/elif/else statements. I think that it's good enough to replace the switch statement.</p>
| 2 | 2008-09-15T22:00:13Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 102,990 | <p>expanding on the "dict as switch" idea. if you want to use a default value for your switch:</p>
<pre><code>def f(x):
try:
return {
'a': 1,
'b': 2,
}[x]
except KeyError:
return 'default'
</code></pre>
| 9 | 2008-09-19T15:37:15Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 103,081 | <p>If you'd like defaults you could use the dictionary <a href="https://docs.python.org/2/library/stdtypes.html#dict.get"><code>get(key[, default])</code></a> method:</p>
<pre><code>def f(x):
return {
'a': 1,
'b': 2,
}.get(x, 9) # 9 is default if x not found
</code></pre>
| 674 | 2008-09-19T15:45:15Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 109,245 | <p>A true <code>switch/case</code> in Python is going to be more difficult than a dictionary method or <code>if/elif/else</code> methods because the simple versions do not support fall through.</p>
<p>Another downfall of the <code>if/elif/else</code> method is the need for repeated comparisons. </p>
<p>The C implementation of a <code>switch/case</code> has a performance benefit over <code>if/else</code> <code>if/else</code> in that only a single comparison is needed. The result of that comparison is used as an offset into a jump table (in the underlying <code>asm</code> generated). </p>
<p>Mimicking the true functionality in Python would be a pain. Does any one have an implementation that would allow for fall through while only using a single comparison?</p>
| 25 | 2008-09-20T20:20:05Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 322,922 | <pre><code>def f(x):
return {'a': 1,'b': 2,}.get(x) or "Default"
</code></pre>
| -2 | 2008-11-27T04:24:36Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 323,259 | <p><a href="http://www.activestate.com/ASPN/Python/Cookbook/">Python Cookbook</a> has several recipes (implementations and corresponding discussions) for switch statement. Please visit the following links:</p>
<ol>
<li><p><a href="http://code.activestate.com/recipes/410692/">Readable switch construction without lambdas or dictionaries</a></p></li>
<li><p><a href="http://code.activestate.com/recipes/410695/">Exception-based Switch-Case</a></p></li>
<li><p><a href="http://code.activestate.com/recipes/181064/">Using a Dictionary in place of a 'switch' statement</a></p></li>
</ol>
| 341 | 2008-11-27T08:53:01Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 956,115 | <p>I have made a (relatively) flexible and re-usable solution for this. It can be found at GitHub as <a href="http://gist.github.com/raw/23036/fd1fb6f05d4c654ff136a9f2545b9f83e9273fcc/switch.py" rel="nofollow">this gist</a>. If the result of the switch function is callable, it is automatically called.</p>
| 4 | 2009-06-05T14:20:52Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 3,129,619 | <p>The solutions I use: </p>
<p>A combination of 2 of the solutions posted here, which is relatively easy to read and supports defaults.</p>
<pre><code>result = {
'a': lambda x: x * 5,
'b': lambda x: x + 7,
'c': lambda x: x - 2
}.get(whatToUse, lambda x: x - 22)(value)
</code></pre>
<p>where</p>
<pre><code>.get('c', lambda x: x - 22)(23)
</code></pre>
<p>looks up <code>"lambda x: x - 2"</code> in the dict and uses it with <code>x=23</code> </p>
<pre><code>.get('xxx', lambda x: x - 22)(44)
</code></pre>
<p>doesn't find it in the dict and uses the default <code>"lambda x: x - 22"</code> with <code>x=44</code>.</p>
| 5 | 2010-06-28T01:32:33Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 3,828,986 | <p>Let's say you don't want to just return a value, but want to use methods that change something on an object. Using the approach stated here would be:</p>
<pre><code>result = {
'a': obj.increment(x),
'b': obj.decrement(x)
}.get(value, obj.default(x))
</code></pre>
<p>What happens here is that python evaluates all methods in the dictionary.
So even if your value is 'a', the object will get incremented <strong>and</strong> decremented by x.</p>
<p>Solution:</p>
<pre><code>func, args = {
'a' : (obj.increment, (x,)),
'b' : (obj.decrement, (x,)),
}.get(value, (obj.default, (x,)))
result = func(*args)
</code></pre>
<p>So you get a list containing a function and its arguments. This way, only the function pointer and the argument list get returned, <em>not</em> evaluated. 'result' then evaluates the returned function call.</p>
| 16 | 2010-09-30T08:31:55Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 4,367,749 | <p>If you're searching extra-statement, as "switch", I built a python module that extends Python. It's called <a href="http://elp.chronocv.fr/?lng=en">ESPY</a> as "Enhanced Structure for Python" and it's available for both Python 2.x and Python 3.x.</p>
<p>For example, in this case, a switch statement could be performed by the following code:</p>
<pre><code>macro switch(arg1):
while True:
cont=False
val=%arg1%
socket case(arg2):
if val==%arg2% or cont:
cont=True
socket
socket else:
socket
break
</code></pre>
<p>that can be used like this:</p>
<pre><code>a=3
switch(a):
case(0):
print("Zero")
case(1):
print("Smaller than 2"):
break
else:
print ("greater than 1")
</code></pre>
<p>so espy translate it in Python as:</p>
<pre><code>a=3
while True:
cont=False
if a==0 or cont:
cont=True
print ("Zero")
if a==1 or cont:
cont=True
print ("Smaller than 2")
break
print ("greater than 1")
break
</code></pre>
| 9 | 2010-12-06T15:08:12Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 6,606,504 | <pre><code>class switch(object):
value = None
def __new__(class_, value):
class_.value = value
return True
def case(*args):
return any((arg == switch.value for arg in args))
</code></pre>
<p>Usage:</p>
<pre><code>while switch(n):
if case(0):
print "You typed zero."
break
if case(1, 4, 9):
print "n is a perfect square."
break
if case(2):
print "n is an even number."
if case(2, 3, 5, 7):
print "n is a prime number."
break
if case(6, 8):
print "n is an even number."
break
print "Only single-digit numbers are allowed."
break
</code></pre>
<p>Tests:</p>
<pre><code>n = 2
#Result:
#n is an even number.
#n is a prime number.
n = 11
#Result:
#Only single-digit numbers are allowed.
</code></pre>
| 56 | 2011-07-07T06:09:09Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 6,606,540 | <p>My favorite one is a really nice <a href="http://code.activestate.com/recipes/410692/">recipe</a>. You'll really like it. It's the closest one I've seen to actual switch case statements, especially in features.</p>
<p>Here's an example:</p>
<pre><code># The following example is pretty much the exact use-case of a dictionary,
# but is included for its simplicity. Note that you can include statements
# in each suite.
v = 'ten'
for case in switch(v):
if case('one'):
print 1
break
if case('two'):
print 2
break
if case('ten'):
print 10
break
if case('eleven'):
print 11
break
if case(): # default, could also just omit condition or 'if True'
print "something else!"
# No need to break here, it'll stop anyway
# break is used here to look as much like the real thing as possible, but
# elif is generally just as good and more concise.
# Empty suites are considered syntax errors, so intentional fall-throughs
# should contain 'pass'
c = 'z'
for case in switch(c):
if case('a'): pass # only necessary if the rest of the suite is empty
if case('b'): pass
# ...
if case('y'): pass
if case('z'):
print "c is lowercase!"
break
if case('A'): pass
# ...
if case('Z'):
print "c is uppercase!"
break
if case(): # default
print "I dunno what c was!"
# As suggested by Pierre Quentel, you can even expand upon the
# functionality of the classic 'case' statement by matching multiple
# cases in a single shot. This greatly benefits operations such as the
# uppercase/lowercase example above:
import string
c = 'A'
for case in switch(c):
if case(*string.lowercase): # note the * for unpacking as arguments
print "c is lowercase!"
break
if case(*string.uppercase):
print "c is uppercase!"
break
if case('!', '?', '.'): # normal argument passing style also applies
print "c is a sentence terminator!"
break
if case(): # default
print "I dunno what c was!"
</code></pre>
| 32 | 2011-07-07T06:12:47Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 10,272,369 | <p>If you have a complicated case block you can consider using a function dictionary lookup table... </p>
<p>If you haven't done this before its a good idea to step into your debugger and view exactly how the dictionary looks up each function. </p>
<p>NOTE: Do <em>not</em> use "()" inside the case/dictionary lookup or it will call each of your functions as the dictionary / case block is created. Remember this because you only want to call each function once using a hash style lookup.</p>
<pre><code>def first_case():
print "first"
def second_case():
print "second"
def third_case():
print "third"
mycase = {
'first': first_case, #do not use ()
'second': second_case, #do not use ()
'third': third_case #do not use ()
}
myfunc = mycase['first']
myfunc()
</code></pre>
| 10 | 2012-04-22T21:43:03Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 12,642,509 | <p>Greg's solutions will not work for unhashable entries. For example when indexing <code>lists</code>.</p>
<pre><code># doesn't work
def give_me_array(key)
return {
[1, 0]: "hello"
}[key]
</code></pre>
<p>Luckily though <code>tuples</code> are hashable.</p>
<pre><code># works
def give_me_array(key)
return {
(1, 0): "hello"
}[tuple(key)]
</code></pre>
<p>Similarly there probably are immutable (thus probably hashable) versions of dictionaries or sets too.</p>
| 0 | 2012-09-28T15:00:11Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 13,239,503 | <pre><code>def f(x):
return 1 if x == 'a' else\
2 if x in 'bcd' else\
0 #default
</code></pre>
<p>Short and easy to read, has a default value and supports expressions in both conditions and return values.</p>
<p>However, it is less efficient than the solution with a dictionary. For example, Python has to scan through all the conditions before returning the default value.</p>
| 4 | 2012-11-05T20:05:12Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 14,688,688 | <p>Just mapping some a key to some code is not really and issue as most people have shown using the dict. The real trick is trying to emulate the whole drop through and break thing. I don't think I've ever written a case statement where I used that "feature". Here's a go at drop through. </p>
<pre><code>def case(list): reduce(lambda b, f: (b | f[0], {False:(lambda:None),True:f[1]}[b | f[0]]())[0], list, False)
case([
(False, lambda:print(5)),
(True, lambda:print(4))
])
</code></pre>
<p>I was really imagining it as a single statement. I hope you'll pardon the silly formatting.</p>
<pre><code>reduce(
initializer=False,
function=(lambda b, f:
( b | f[0]
, { False: (lambda:None)
, True : f[1]
}[b | f[0]]()
)[0]
),
iterable=[
(False, lambda:print(5)),
(True, lambda:print(4))
]
)
</code></pre>
<p>I hope that's valid python. It should give you drop through. of course the boolean checks could be expressions and if you wanted them to be evaluated lazily you could wrap them all in a lambda. I wouldn't be to hard to make it accept after executing some of the items in the list either. Just make the tuple (bool, bool, function) where the second bool indicates whether or not to break or drop through.</p>
| 0 | 2013-02-04T14:21:46Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 16,964,129 | <p>also use the List for store the cases ,and call corresponding function by select-</p>
<pre><code>cases = ['zero()','one()','two()','three()']
def zero():
print "method for 0 called..."
def one():
print "method for 1 called..."
def two():
print "method for 2 called..."
def three():
print "method for 3 called..."
i = int(raw_input("Enter choice between 0-3 "))
if(i<=len(cases)):
exec(cases[i])
else:
print "wrong choice"
</code></pre>
<p>also explained at <a href="http://screwdesk.com/python-switch-case-alternative/" rel="nofollow">screwdesk</a> </p>
| 0 | 2013-06-06T14:01:46Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 17,865,871 | <p>Defining:</p>
<pre><code>def switch1(value, options):
if value in options:
options[value]()
</code></pre>
<p>allows you to use a fairly straightforward syntax, with the cases bundled into a map:</p>
<pre><code>def sample1(x):
local = 'betty'
switch1(x, {
'a': lambda: print("hello"),
'b': lambda: (
print("goodbye," + local),
print("!")),
})
</code></pre>
<p>I kept trying to redefine switch in a way that would let me get rid of the "lambda:", but gave up. Tweaking the definition:</p>
<pre><code>def switch(value, *maps):
options = {}
for m in maps:
options.update(m)
if value in options:
options[value]()
elif None in options:
options[None]()
</code></pre>
<p>Allowed me to map multiple cases to the same code, and to supply a default option:</p>
<pre><code>def sample(x):
switch(x, {
_: lambda: print("other")
for _ in 'cdef'
}, {
'a': lambda: print("hello"),
'b': lambda: (
print("goodbye,"),
print("!")),
None: lambda: print("I dunno")
})
</code></pre>
<p>Each replicated case has to be in its own dictionary; switch() consolidates the dictionaries before looking up the value. It's still uglier than I'd like, but it has the basic efficiency of using a hashed lookup on the expression, rather than a loop through all the keys.</p>
| 3 | 2013-07-25T18:23:33Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 19,335,626 | <p>I didn't find the simple answer I was looking for anywhere on Google search. But I figured it out anyway. It's really quite simple. Decided to post it, and maybe prevent a few less scratches on someone else's head. The key is simply "in" and tuples. Here is the switch statement behavior with fall-through, including RANDOM fall-through.</p>
<pre><code>l = ['Dog', 'Cat', 'Bird', 'Bigfoot',
'Dragonfly', 'Snake', 'Bat', 'Loch Ness Monster']
for x in l:
if x in ('Dog', 'Cat'):
x += " has four legs"
elif x in ('Bat', 'Bird', 'Dragonfly'):
x += " has wings."
elif x in ('Snake',):
x += " has a forked tongue."
else:
x += " is a big mystery by default."
print(x)
print()
for x in range(10):
if x in (0, 1):
x = "Values 0 and 1 caught here."
elif x in (2,):
x = "Value 2 caught here."
elif x in (3, 7, 8):
x = "Values 3, 7, 8 caught here."
elif x in (4, 6):
x = "Values 4 and 6 caught here"
else:
x = "Values 5 and 9 caught in default."
print(x)
</code></pre>
<p>Provides:</p>
<pre><code>Dog has four legs
Cat has four legs
Bird has wings.
Bigfoot is a big mystery by default.
Dragonfly has wings.
Snake has a forked tongue.
Bat has wings.
Loch Ness Monster is a big mystery by default.
Values 0 and 1 caught here.
Values 0 and 1 caught here.
Value 2 caught here.
Values 3, 7, 8 caught here.
Values 4 and 6 caught here
Values 5 and 9 caught in default.
Values 4 and 6 caught here
Values 3, 7, 8 caught here.
Values 3, 7, 8 caught here.
Values 5 and 9 caught in default.
</code></pre>
| 7 | 2013-10-12T15:04:01Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 27,212,138 | <p>I liked <a href="http://stackoverflow.com/a/60215/1766716">Mark Bies's answer</a>, but I am getting error. So modified it to run in a comprehensible way.</p>
<pre><code>In [1]: result = {
...: 'a': lambda x: 'A',
...: 'b': lambda x: 'B',
...: 'c': lambda x: 'C'
...: }['a'](x)
---------------------------------------------------------------------------
NameError Traceback (most recent call last)
<ipython-input-79-5ce2b3ae3711> in <module>()
3 'b': lambda x: 'B',
4 'c': lambda x: 'C'
----> 5 }['a'](x)
NameError: name 'x' is not defined
</code></pre>
<p><strong>Input 2 works but in a weird way. I have to run with</strong> <code>results[value](value)</code></p>
<pre><code>In [2]: result = {
...: 'a': lambda x: 'A',
...: 'b': lambda x: 'B',
...: 'c': lambda x: 'C'
...: }
...: result['a']('a')
...:
Out[2]: 'A'
</code></pre>
<p><strong>Input 3 works in a comprehensible way. I use this with</strong> <code>result[value]()</code></p>
<pre><code>In [3]: result = {
...: 'a': lambda : 'A',
...: 'b': lambda : 'B',
...: 'c': lambda : 'C',
...: None: lambda : 'Nothing else matters'
...: }
...: result['a']()
...:
Out[3]: 'A'
</code></pre>
<p><strong>Edit:</strong> I noticed that i can use <code>None</code> type with with dictionaries. So this would emulate <code>switch ; case else</code></p>
| 2 | 2014-11-30T10:12:36Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 27,746,465 | <p>I found that a common switch structure:</p>
<pre><code>switch ...parameter...
case p1: v1; break;
case p2: v2; break;
default: v3;
</code></pre>
<p>can be expressed in Python as follows:</p>
<pre><code>(lambda x: v1 if p1(x) else v2 if p2(x) else v3)
</code></pre>
<p>or formatted in a clearer way:</p>
<pre><code>(lambda x:
v1 if p1(x) else
v2 if p2(x) else
v3)
</code></pre>
<p>Instead of being a statement, the python version is an expression, which evaluates to a value.</p>
| 7 | 2015-01-02T18:01:25Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 30,012,053 | <pre><code>class Switch:
def __init__(self, value): self._val = value
def __enter__(self): return self
def __exit__(self, type, value, traceback): return False # Allows traceback to occur
def __call__(self, cond, *mconds): return self._val in (cond,)+mconds
from datetime import datetime
with Switch(datetime.today().weekday()) as case:
if case(0):
# Basic usage of switch
print("I hate mondays so much.")
# Note there is no break needed here
elif case(1,2):
# This switch also supports multiple conditions (in one line)
print("When is the weekend going to be here?")
elif case(3,4): print("The weekend is near.")
else:
# Default would occur here
print("Let's go have fun!") # Didn't use case for example purposes
</code></pre>
| 8 | 2015-05-03T09:05:43Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 30,881,320 | <p>My favorite Python recipe for switch/case is:</p>
<pre><code>choices = {'a': 1, 'b': 2}
result = choices.get(key, 'default')
</code></pre>
<p>Short and simple for simple scenarios. </p>
<p>Compare to 11+ lines of C code:</p>
<pre><code>// C Language version of a simple 'switch/case'.
switch( key )
{
case 'a' :
result = 1;
break;
case 'b' :
result = 2;
break;
default :
result = -1;
}
</code></pre>
<p>You can even assign multiple variables by using tuples:</p>
<pre><code>choices = {'a': (1, 2, 3), 'b': (4, 5, 6)}
(result1, result2, result3) = choices.get(key, ('default1', 'default2', 'default3'))
</code></pre>
| 34 | 2015-06-17T02:25:10Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 31,995,013 | <pre><code># simple case alternative
some_value = 5.0
# this while loop block simulates a case block
# case
while True:
# case 1
if some_value > 5:
print ('Greater than five')
break
# case 2
if some_value == 5:
print ('Equal to five')
break
# else case 3
print ( 'Must be less than 5')
break
</code></pre>
| 2 | 2015-08-13T17:40:47Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 34,639,742 | <p>For the sake of completeness, here are some of my attempts back in stone-age:</p>
<p><a href="http://code.activestate.com/recipes/269708-some-python-style-switches/?in=user-1521341" rel="nofollow">http://code.activestate.com/recipes/269708-some-python-style-switches/?in=user-1521341</a></p>
<p>I especially enjoy the use of "3. Select values with 'range comparisons'"</p>
| 0 | 2016-01-06T18:05:24Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 36,079,070 | <p>I think the best way is to <strong>use the python language idioms to keep your code testable</strong>. As showed in previous answers, I use dictionaries to <strong>take advantage of python structures and language</strong> and keep the "case" code isolated in different methods. Below there is a class, but you can use directly a module, globals and functions. The class has methods that <strong>can be tested with isolation</strong>.
Dependending to your needs, you can play with static methods and attributes too.</p>
<pre><code>class ChoiceManager:
def __init__(self):
self.__choice_table = \
{
"CHOICE1" : self.my_func1,
"CHOICE2" : self.my_func2,
}
def my_func1(self, data):
pass
def my_func2(self, data):
pass
def process(self, case, data):
return self.__choice_table[case](data)
ChoiceManager().process("CHOICE1", my_data)
</code></pre>
<p>It is possible to <strong>take advantage of this method using also classes as keys</strong> of "__choice_table". In this way you can avoid <em>isinstance abuse</em> and keep all clean and testable.</p>
<p>Supposing you have to process a lot of messages or packets from the net or your MQ. Every packet has its own structure and its management code (in a generic way).
With the above code it is possible to do something like this:</p>
<pre><code>class PacketManager:
def __init__(self):
self.__choice_table = \
{
ControlMessage : self.my_func1,
DiagnosticMessage : self.my_func2,
}
def my_func1(self, data):
# process the control message here
pass
def my_func2(self, data):
# process the diagnostic message here
pass
def process(self, pkt):
return self.__choice_table[pkt.__class__](pkt)
pkt = GetMyPacketFromNet()
PacketManager().process(pkt)
# isolated test or isolated usage example
def test_control_packet():
p = ControlMessage()
PacketManager().my_func1(p)
</code></pre>
<p>So <strong>complexity is not spread in the code flow but it is rendered in code structure</strong>.</p>
| 1 | 2016-03-18T08:01:35Z | [
"python",
"switch-statement"
] |
Replacements for switch statement in Python? | 60,208 | <p>I want to write a function in Python that returns different fixed values based on the value of an input index. </p>
<p>In other languages I would use a <code>switch</code> or <code>case</code> statement, but Python does not appear to have a <code>switch</code> statement. What are the recommended Python solutions in this scenario?</p>
| 788 | 2008-09-13T00:36:30Z | 37,448,954 | <p>If you don't worry losing syntax highlight inside the case suites, you can do the following:</p>
<pre><code>exec {
1: """
print ('one')
""",
2: """
print ('two')
""",
3: """
print ('three')
""",
}.get(value, """
print ('None')
""")
</code></pre>
<p>Where <code>value</code> is the value. In C, this would be:</p>
<pre><code>switch (value) {
case 1:
printf("one");
break;
case 2:
printf("two");
break;
case 3:
printf("three");
break;
default:
printf("None");
break;
}
</code></pre>
<p>We can also create a helper function to do this:</p>
<pre><code>def switch(value, cases, default):
exec cases.get(value, default)
</code></pre>
<p>So we can use it like this for the example with one, two and three:</p>
<pre><code>switch(value, {
1: """
print ('one')
""",
2: """
print ('two')
""",
3: """
print ('three')
""",
}, """
print ('None')
""")
</code></pre>
| 0 | 2016-05-25T22:56:30Z | [
"python",
"switch-statement"
] |
Can distutils create empty __init__.py files? | 60,352 | <p>If all of my <code>__init__.py</code> files are empty, do I have to store them into version control, or is there a way to make <code>distutils</code> create empty <code>__init__.py</code> files during installation?</p>
| 0 | 2008-09-13T05:51:17Z | 60,431 | <p>In Python, <code>__init__.py</code> files actually have a meaning! They mean that the folder they are in is a Python module. As such, they have a real role in your code and should most probably be stored in Version Control.</p>
<p>You could well imagine a folder in your source tree that is NOT a Python module, for example a folder containing only resources (e.g. images) and no code. That folder would not need to have a <code>__init__.py</code> file in it. Now how do you make the difference between folders where distutils should create those files and folders where it should not ?</p>
| 7 | 2008-09-13T09:19:56Z | [
"python",
"version-control",
"distutils"
] |
Can distutils create empty __init__.py files? | 60,352 | <p>If all of my <code>__init__.py</code> files are empty, do I have to store them into version control, or is there a way to make <code>distutils</code> create empty <code>__init__.py</code> files during installation?</p>
| 0 | 2008-09-13T05:51:17Z | 60,506 | <p>Is there a reason you want to <em>avoid</em> putting empty <code>__init__.py</code> files in version control? If you do this you won't be able to <code>import</code> your packages from the source directory wihout first running distutils.</p>
<p>If you really want to, I suppose you can create <code>__init__.py</code> in <code>setup.py</code>. It has to be <em>before</em> running <code>distutils.setup</code>, so <code>setup</code> itself is able to find your packages:</p>
<pre><code>from distutils import setup
import os
for path in [my_package_directories]:
filename = os.path.join(pagh, '__init__.py')
if not os.path.exists(filename):
init = open(filename, 'w')
init.close()
setup(
...
)
</code></pre>
<p>but... what would you gain from this, compared to having the empty <code>__init__.py</code> files there in the first place? </p>
| 4 | 2008-09-13T12:46:50Z | [
"python",
"version-control",
"distutils"
] |
Windows Mobile development in Python | 60,446 | <p>What is the best way to start developing Windows Mobile Professional applications in Python? Is there a reasonable SDK including an emulator? Is it even possible without doing excessive amount of underlaying Windows API calls for UI for instance?</p>
| 3 | 2008-09-13T09:56:04Z | 60,448 | <p><H2>Python CE</H2>
Python port for Windows CE (Pocket PC) devices. Intended to be as close to desktop version as possible (console, current directory support, testsuite passed). </p>
<p><a href="http://pythonce.sourceforge.net/"><img src="http://pythonce.sourceforge.net/images/python-logo.jpg" alt="Python CE" /></a></p>
<p><img src="http://sourceforge.net/dbimage.php?id=76454" alt="alt text" /></p>
| 9 | 2008-09-13T10:03:55Z | [
"python",
"windows-mobile"
] |
Windows Mobile development in Python | 60,446 | <p>What is the best way to start developing Windows Mobile Professional applications in Python? Is there a reasonable SDK including an emulator? Is it even possible without doing excessive amount of underlaying Windows API calls for UI for instance?</p>
| 3 | 2008-09-13T09:56:04Z | 60,460 | <p>If the IronPython and .Net Compact Framework teams work together, Visual Studio may one day support Python for Windows Mobile development out-of-the-box. Unfortunately, <a href="http://www.codeplex.com/IronPython/WorkItem/View.aspx?WorkItemId=9191" rel="nofollow">this feature request has been sitting on their issue tracker for ages</a>...</p>
| 1 | 2008-09-13T10:29:26Z | [
"python",
"windows-mobile"
] |
Windows Mobile development in Python | 60,446 | <p>What is the best way to start developing Windows Mobile Professional applications in Python? Is there a reasonable SDK including an emulator? Is it even possible without doing excessive amount of underlaying Windows API calls for UI for instance?</p>
| 3 | 2008-09-13T09:56:04Z | 60,945 | <p>(I used to write customer apps for Windows Mobile.)</p>
<p>Forget about python. Even if it's technically possible:</p>
<ul>
<li>your app will be big (you'll have to bundle the whole python runtime with your app)</li>
<li>your app will use lots of memory (python is a memory hog, relative to C/C++)</li>
<li>your app will be slow</li>
<li>you wont find any documentation or discussion groups to help you when you (inevitably) encounter problems</li>
</ul>
<p>Go with C/C++ (or C#). Visual Studio 2005/2008 have decent tools for those (SDK for winmo built-in, debugging on the emulator or device connected through USB), the best documentation is for those technologies plus there are active forums/discussion groups/mailing lists where you can ask for help.</p>
| 9 | 2008-09-13T22:30:01Z | [
"python",
"windows-mobile"
] |
Windows Mobile development in Python | 60,446 | <p>What is the best way to start developing Windows Mobile Professional applications in Python? Is there a reasonable SDK including an emulator? Is it even possible without doing excessive amount of underlaying Windows API calls for UI for instance?</p>
| 3 | 2008-09-13T09:56:04Z | 1,519,440 | <p>Just found this: <a href="http://ejr44.blogspot.com/2008/05/python-for-windows-mobile-cab.html" rel="nofollow">http://ejr44.blogspot.com/2008/05/python-for-windows-mobile-cab.html</a></p>
<p>Looks like a complete set of .CAB files to provide Python on Windows Mobile.</p>
| 1 | 2009-10-05T11:07:39Z | [
"python",
"windows-mobile"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 60,660 | <p>For C++, check out <a href="http://www.boost.org/doc/libs/1_36_0/doc/html/interprocess.html">Boost IPC</a>.<br />
You can probably create or find some bindings for the scripting languages as well. </p>
<p>Otherwise if it's really important to be able to interface with scripting languages your best bet is simply to use files, pipes or sockets or even a higher level abstraction like HTTP. </p>
| 13 | 2008-09-13T16:19:10Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 60,662 | <p>How about <a href="http://incubator.apache.org/thrift/">Facebook's Thrift</a>?</p>
<blockquote>
<p>Thrift is a software framework for scalable cross-language services development. It combines a software stack with a code generation engine to build services that work efficiently and seamlessly between C++, Java, Python, PHP, Ruby, Erlang, Perl, Haskell, C#, Cocoa, Smalltalk, and OCaml.</p>
</blockquote>
| 5 | 2008-09-13T16:21:24Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 60,668 | <p>It doesn't get more simple than using pipes, which are supported on every OS I know of, and can be accessed in pretty much every language.</p>
<p>Check out <a href="http://web.archive.org/web/20080919054639/http://www.utdallas.edu/~kcooper/teaching/3375/Tutorial6a/tutorial6.htm" rel="nofollow">this</a> tutorial.</p>
| 4 | 2008-09-13T16:27:20Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 60,692 | <p>TCP sockets to localhost FTW.</p>
| 2 | 2008-09-13T17:03:28Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 60,702 | <p>I think you'll want something based on sockets. </p>
<p>If you want RPC rather than just IPC I would suggest something like XML-RPC/SOAP which runs over HTTP, and can be used from any language.</p>
| 5 | 2008-09-13T17:17:20Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 65,924 | <p>In terms of speed, the best cross-platform IPC mechanism will be pipes. That assumes, however, that you want cross-platform IPC on the same machine. If you want to be able to talk to processes on remote machines, you'll want to look at using sockets instead. Luckily, if you're talking about TCP at least, sockets and pipes behave pretty much the same behavior. While the APIs for setting them up and connecting them are different, they both just act like streams of data.</p>
<p>The difficult part, however, is not the communication channel, but the messages you pass over it. You really want to look at something that will perform verification and parsing for you. I recommend looking at Google's <a href="http://code.google.com/apis/protocolbuffers/docs/overview.html">Protocol Buffers</a>. You basically create a spec file that describes the object you want to pass between processes, and there is a compiler that generates code in a number of different languages for reading and writing objects that match the spec. It's much easier (and less bug prone) than trying to come up with a messaging protocol and parser yourself.</p>
| 42 | 2008-09-15T19:22:37Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 66,069 | <p>If you're willing to try something a little different, there's the <a href="http://zeroc.com/ice.html" rel="nofollow">ICE</a> platform from <a href="http://zeroc.com" rel="nofollow">ZeroC</a>. It's open source, and is supported on pretty much every OS you can think of, as well as having language support for C++, C#, Java, Ruby, Python and PHP. Finally, it's very easy to drive (the language mappings are tailored to fit naturally into each language). It's also fast and efficient. There's even a cut-down version for devices.</p>
| 4 | 2008-09-15T19:39:28Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 67,532 | <p>You might want to try <a href="http://www.msobczak.com/prog/yami/">YAMI</a> , it's very simple yet functional, portable and comes with binding to few languages</p>
| 7 | 2008-09-15T22:11:03Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 74,590 | <p>Why not D-Bus? It's a very simple message passing system that runs on almost all platforms and is designed for robustness. It's supported by pretty much every scripting language at this point.</p>
<p><a href="http://freedesktop.org/wiki/Software/dbus">http://freedesktop.org/wiki/Software/dbus</a></p>
| 8 | 2008-09-16T17:04:46Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 74,615 | <p>Python has a pretty good IPC library: see <a href="https://docs.python.org/2/library/ipc.html" rel="nofollow"><a href="https://docs.python.org/2/library/ipc.html" rel="nofollow">https://docs.python.org/2/library/ipc.html</a></a></p>
| 0 | 2008-09-16T17:07:42Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 351,256 | <p>Distributed computing is usually complex and you are well advised to use existing libraries or frameworks instead of reinventing the wheel. Previous poster have already enumerated a couple of these libraries and frameworks. Depending on your needs you can pick either a very low level (like sockets) or high level framework (like CORBA). There can not be a generic "use this" answer. You need to educate yourself about distributed programming and then will find it much easier to pick the right library or framework for the job.</p>
<p>There exists a wildly used C++ framework for distributed computing called ACE and the CORBA ORB TAO (which is buildt upon ACE). There exist very good books about ACE <a href="http://www.cs.wustl.edu/~schmidt/ACE/" rel="nofollow">http://www.cs.wustl.edu/~schmidt/ACE/</a> so you might take a look. Take care!</p>
| 3 | 2008-12-08T22:55:24Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 2,136,606 | <p><a href="http://www.msobczak.com/prog/yami/" rel="nofollow">YAMI - Yet Another Messaging Infrastructure</a> is a lightweight messaging and networking framework.</p>
| 3 | 2010-01-25T23:52:16Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 5,375,042 | <p>You might want to check out <a href="http://www.angryredplanet.com/~hackbod/openbinder/docs/html/" rel="nofollow">openbinder</a>.</p>
| 0 | 2011-03-21T07:52:53Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 19,100,365 | <p>google protobufs are a really bad idea with you want easy to maintain and debug code. its too easy for people to abuse it and use it to pollute your code. the proto files are nice, but its basically the same thing as a structure header file, and the code it generates is complete crap making you wonder if it really a covert attack tool to sabotage software projects instead of automating them. After you use it for a while its almost impossible to remove it from your code. you are better off just using a header file of fix format structures that are easily debugged. </p>
<p>if you really need compression, switch to an address/data mapping of filing structures remotely...
then packets are just a bundle of address/data pairs... also a structure that is very easy to automate with your own perl scripts that produce code that is human readable and debugable </p>
| -2 | 2013-09-30T17:37:19Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 25,034,401 | <p>I would use TCP sockets as other have suggested.</p>
<p>But to add something different, if you want a portable, easy to use, multi-language and <a href="http://zeromq.org/intro:commercial-support" rel="nofollow">LGPL</a>ed solution, I would recommend you <a href="http://zeromq.org/" rel="nofollow">ZeroMQ</a>. ZeroMQ + Protocol Buffers (which other have already mentioned as well) is a powerfull combination!</p>
<p>ZeroMQ is amazingly fast and simple. Suitable for simple and complex systems/architectures. You'll need to try it to believe it.</p>
| 1 | 2014-07-30T10:22:40Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
cross platform IPC | 60,649 | <p>I'm looking for suggestions on possible IPC mechanisms that are:</p>
<ul>
<li><strong>cross platform</strong> (WIN32 and Linux at least)</li>
<li>Simple to implement in <strong>C++</strong> as well as the <strong>most common scripting languages</strong> (perl, ruby python etc).</li>
<li>Finally, <strong>simple to use</strong> from a programming point of view!</li>
</ul>
<p>What are my options? I'm programming under Linux, but I'd like what I write to be portable to other OSes in the future. I've thought about using sockets, named pipes, or something like DBus.</p>
<p>Does anyone have any advice?</p>
| 53 | 2008-09-13T16:10:34Z | 37,910,705 | <p>I can suggest you to use the <a href="https://github.com/saprykin/plibsys" rel="nofollow">plibsys</a> C library. It is very simple, lightweight and cross-platform. Released under the LGPL. It provides:</p>
<ul>
<li>named system-wide shared memory regions (System V, POSIX and Windows implementations);</li>
<li>named system-wide semaphores for access synchronization (System V, POSIX and Windows implementations);</li>
<li>named system-wide shared buffer implementation based on the shared memory and semaphore;</li>
<li>sockets (TCP, UDP, SCTP) with IPv4 and IPv6 support (UNIX and Windows implementations).</li>
</ul>
<p>It is easy to use library with quite a good documentation. As it is written in C you can easily make bindings from scripting languages.</p>
<p>If you need to pass large data sets between processes (especially if speed is essential) it is better to use shared memory to pass the data itself and sockets to notify a process that the data is ready. You can make it as following:</p>
<ul>
<li>a process puts the data into a shared memory segment and sends a notification via a socket to another process; as a notification usually is very small the time overhead is minimal;</li>
<li>another process receives the notification and reads the data from the shared memory segment; after that it sends a notification that the data was read back to the first process so it can feed more data.</li>
</ul>
<p>This approach can be implemented in a cross-platform fashion.</p>
| 0 | 2016-06-19T18:54:44Z | [
"c++",
"python",
"linux",
"cross-platform",
"ipc"
] |
How do I write a python HTTP server to listen on multiple ports? | 60,680 | <p>I'm writing a small web server in Python, using BaseHTTPServer and a custom subclass of BaseHTTPServer.BaseHTTPRequestHandler. Is it possible to make this listen on more than one port?</p>
<p>What I'm doing now:</p>
<pre><code>class MyRequestHandler(BaseHTTPServer.BaseHTTPRequestHandler):
def doGET
[...]
class ThreadingHTTPServer(ThreadingMixIn, HTTPServer):
pass
server = ThreadingHTTPServer(('localhost', 80), MyRequestHandler)
server.serve_forever()
</code></pre>
| 14 | 2008-09-13T16:42:38Z | 60,753 | <p>Sure; just start two different servers on two different ports in two different threads that each use the same handler. Here's a complete, working example that I just wrote and tested. If you run this code then you'll be able to get a Hello World webpage at both <a href="http://localhost:1111/">http://localhost:1111/</a> and <a href="http://localhost:2222/">http://localhost:2222/</a></p>
<pre><code>from threading import Thread
from SocketServer import ThreadingMixIn
from BaseHTTPServer import HTTPServer, BaseHTTPRequestHandler
class Handler(BaseHTTPRequestHandler):
def do_GET(self):
self.send_response(200)
self.send_header("Content-type", "text/plain")
self.end_headers()
self.wfile.write("Hello World!")
class ThreadingHTTPServer(ThreadingMixIn, HTTPServer):
pass
def serve_on_port(port):
server = ThreadingHTTPServer(("localhost",port), Handler)
server.serve_forever()
Thread(target=serve_on_port, args=[1111]).start()
serve_on_port(2222)
</code></pre>
| 27 | 2008-09-13T18:22:06Z | [
"python",
"webserver"
] |
How do I write a python HTTP server to listen on multiple ports? | 60,680 | <p>I'm writing a small web server in Python, using BaseHTTPServer and a custom subclass of BaseHTTPServer.BaseHTTPRequestHandler. Is it possible to make this listen on more than one port?</p>
<p>What I'm doing now:</p>
<pre><code>class MyRequestHandler(BaseHTTPServer.BaseHTTPRequestHandler):
def doGET
[...]
class ThreadingHTTPServer(ThreadingMixIn, HTTPServer):
pass
server = ThreadingHTTPServer(('localhost', 80), MyRequestHandler)
server.serve_forever()
</code></pre>
| 14 | 2008-09-13T16:42:38Z | 60,754 | <p>Not easily. You could have two ThreadingHTTPServer instances, write your own serve_forever() function (don't worry it's not a complicated function).</p>
<p>The existing function:</p>
<pre><code>def serve_forever(self, poll_interval=0.5):
"""Handle one request at a time until shutdown.
Polls for shutdown every poll_interval seconds. Ignores
self.timeout. If you need to do periodic tasks, do them in
another thread.
"""
self.__serving = True
self.__is_shut_down.clear()
while self.__serving:
# XXX: Consider using another file descriptor or
# connecting to the socket to wake this up instead of
# polling. Polling reduces our responsiveness to a
# shutdown request and wastes cpu at all other times.
r, w, e = select.select([self], [], [], poll_interval)
if r:
self._handle_request_noblock()
self.__is_shut_down.set()
</code></pre>
<p>So our replacement would be something like:</p>
<pre><code>def serve_forever(server1,server2):
while True:
r,w,e = select.select([server1,server2],[],[],0)
if server1 in r:
server1.handle_request()
if server2 in r:
server2.handle_request()
</code></pre>
| 4 | 2008-09-13T18:23:10Z | [
"python",
"webserver"
] |
How do I write a python HTTP server to listen on multiple ports? | 60,680 | <p>I'm writing a small web server in Python, using BaseHTTPServer and a custom subclass of BaseHTTPServer.BaseHTTPRequestHandler. Is it possible to make this listen on more than one port?</p>
<p>What I'm doing now:</p>
<pre><code>class MyRequestHandler(BaseHTTPServer.BaseHTTPRequestHandler):
def doGET
[...]
class ThreadingHTTPServer(ThreadingMixIn, HTTPServer):
pass
server = ThreadingHTTPServer(('localhost', 80), MyRequestHandler)
server.serve_forever()
</code></pre>
| 14 | 2008-09-13T16:42:38Z | 61,322 | <p>I would say that threading for something this simple is overkill. You're better off using some form of asynchronous programming.</p>
<p>Here is an example using <a href="http://twistedmatrix.com/">Twisted</a>:</p>
<pre><code>from twisted.internet import reactor
from twisted.web import resource, server
class MyResource(resource.Resource):
isLeaf = True
def render_GET(self, request):
return 'gotten'
site = server.Site(MyResource())
reactor.listenTCP(8000, site)
reactor.listenTCP(8001, site)
reactor.run()
</code></pre>
<p>I also thinks it looks a lot cleaner to have each port be handled in the same way, instead of having the main thread handle one port and an additional thread handle the other. Arguably that can be fixed in the thread example, but then you're using three threads.</p>
| 6 | 2008-09-14T13:15:51Z | [
"python",
"webserver"
] |
python regex to match multi-line preprocessor macro | 60,685 | <p>What follows is a regular expression I have written to match multi-line pre-processor macros in C / C++ code. I'm by no means a regular expressions guru, so I'd welcome any advice on how I can make this better.</p>
<p>Here's the regex:</p>
<pre><code>\s*#define(.*\\\n)+[\S]+(?!\\)
</code></pre>
<p>It should match all of this:</p>
<pre><code>#define foo(x) if(x) \
doSomething(x)
</code></pre>
<p>But only some of this (shouldn't match the next line of code:</p>
<pre><code>#define foo(x) if(x) \
doSomething(x)
normalCode();
</code></pre>
<p>And also shouldn't match single-line preprocessor macros.</p>
<p>I'm pretty sure that the regex above works - but as I said, there probably a better way of doing it, and I imagine that there are ways of breaking it. Can anyone suggest any?</p>
| 5 | 2008-09-13T16:53:57Z | 60,723 | <p>This is a simple test program I knocked up:</p>
<pre><code>#!/usr/bin/env python
TEST1="""
#include "Foo.h"
#define bar foo\\
x
#include "Bar.h"
"""
TEST2="""
#define bar foo
#define x 1 \\
12 \\
2 \\\\ 3
Foobar
"""
TEST3="""
#define foo(x) if(x) \\
doSomething(x)
"""
TEST4="""
#define foo(x) if(x) \\
doSomething(x)
normalCode();
"""
import re
matcher = re.compile(r"^[ \t]*#define(.*\\\n)+.*$",re.MULTILINE)
def extractDefines(s):
mo = matcher.search(s)
if not mo:
print mo
return
print mo.group(0)
extractDefines(TEST1)
extractDefines(TEST2)
extractDefines(TEST3)
extractDefines(TEST4)
</code></pre>
<p>The re I used:</p>
<pre><code>r"^[ \t]*#define(.*\\\n)+.*$"
</code></pre>
<p>Is very similar to the one use used, the changes:</p>
<ol>
<li>[ \t] To avoid newlines at the start
of the define.</li>
<li>I rely on + being
greedy, so I can use a simple .*$ at
the end to get the first line of the
define that doesn't end with \</li>
</ol>
| 5 | 2008-09-13T17:46:14Z | [
"python",
"regex"
] |
python regex to match multi-line preprocessor macro | 60,685 | <p>What follows is a regular expression I have written to match multi-line pre-processor macros in C / C++ code. I'm by no means a regular expressions guru, so I'd welcome any advice on how I can make this better.</p>
<p>Here's the regex:</p>
<pre><code>\s*#define(.*\\\n)+[\S]+(?!\\)
</code></pre>
<p>It should match all of this:</p>
<pre><code>#define foo(x) if(x) \
doSomething(x)
</code></pre>
<p>But only some of this (shouldn't match the next line of code:</p>
<pre><code>#define foo(x) if(x) \
doSomething(x)
normalCode();
</code></pre>
<p>And also shouldn't match single-line preprocessor macros.</p>
<p>I'm pretty sure that the regex above works - but as I said, there probably a better way of doing it, and I imagine that there are ways of breaking it. Can anyone suggest any?</p>
| 5 | 2008-09-13T16:53:57Z | 60,729 | <pre><code>start = r"^\s*#define\s+"
continuation = r"(?:.*\\\n)+"
lastline = r".*$"
re_multiline_macros = re.compile(start + continuation + lastline,
re.MULTILINE)
</code></pre>
| 3 | 2008-09-13T17:53:30Z | [
"python",
"regex"
] |
Getting random row through SQLAlchemy | 60,805 | <p>How do I select a(or some) random row(s) from a table using SQLAlchemy? </p>
| 49 | 2008-09-13T19:58:02Z | 60,811 | <p>Theres a couple of ways through SQL, depending on which data base is being used.</p>
<p>(I think SQLAlchemy can use all these anyways)</p>
<p>mysql:</p>
<pre><code>SELECT colum FROM table
ORDER BY RAND()
LIMIT 1
</code></pre>
<p>PostgreSQL:</p>
<pre><code>SELECT column FROM table
ORDER BY RANDOM()
LIMIT 1
</code></pre>
<p>MSSQL:</p>
<pre><code>SELECT TOP 1 column FROM table
ORDER BY NEWID()
</code></pre>
<p>IBM DB2:</p>
<pre><code>SELECT column, RAND() as IDX
FROM table
ORDER BY IDX FETCH FIRST 1 ROWS ONLY
</code></pre>
<p>Oracle:</p>
<pre><code>SELECT column FROM
(SELECT column FROM table
ORDER BY dbms_random.value)
WHERE rownum = 1
</code></pre>
<p>However I don't know of any standard way</p>
| -4 | 2008-09-13T20:04:11Z | [
"python",
"sql",
"database",
"sqlalchemy"
] |
Getting random row through SQLAlchemy | 60,805 | <p>How do I select a(or some) random row(s) from a table using SQLAlchemy? </p>
| 49 | 2008-09-13T19:58:02Z | 60,815 | <p>This is very much a database-specific issue.</p>
<p>I know that PostgreSQL and MySQL have the ability to order by a random function, so you can use this in SQLAlchemy:</p>
<pre><code>from sqlalchemy.sql.expression import func, select
select.order_by(func.random()) # for PostgreSQL, SQLite
select.order_by(func.rand()) # for MySQL
select.order_by('dbms_random.value') # For Oracle
</code></pre>
<p>Next, you need to limit the query by the number of records you need (for example using <code>.limit()</code>).</p>
<p>Bear in mind that at least in PostgreSQL, selecting random record has severe perfomance issues; <a href="http://www.depesz.com/index.php/2007/09/16/my-thoughts-on-getting-random-row/">here</a> is good article about it.</p>
| 63 | 2008-09-13T20:09:28Z | [
"python",
"sql",
"database",
"sqlalchemy"
] |
Getting random row through SQLAlchemy | 60,805 | <p>How do I select a(or some) random row(s) from a table using SQLAlchemy? </p>
| 49 | 2008-09-13T19:58:02Z | 390,676 | <p>If you are using the orm and the table is not big (or you have its amount of rows cached) and you want it to be database independent the really simple approach is.</p>
<pre><code>import random
rand = random.randrange(0, session.query(Table).count())
row = session.query(Table)[rand]
</code></pre>
<p>This is cheating slightly but thats why you use an orm.</p>
| 20 | 2008-12-24T03:22:30Z | [
"python",
"sql",
"database",
"sqlalchemy"
] |
Getting random row through SQLAlchemy | 60,805 | <p>How do I select a(or some) random row(s) from a table using SQLAlchemy? </p>
| 49 | 2008-09-13T19:58:02Z | 782,514 | <p>An enhanced version of Lukasz's example, in the case you need to select multiple rows at random:</p>
<pre><code>import random
# you must first select all the values of the primary key field for the table.
# in some particular cases you can use xrange(session.query(Table).count()) instead
ids = session.query(Table.primary_key_field).all()
ids_sample = random.sample(ids, 100)
rows = session.query(Table).filter(Table.primary_key_field.in_(ids_sample))
</code></pre>
<p>So, this post is just to point out that you can use .in_ to select multiple fields at the same time.</p>
| -2 | 2009-04-23T16:27:26Z | [
"python",
"sql",
"database",
"sqlalchemy"
] |
Getting random row through SQLAlchemy | 60,805 | <p>How do I select a(or some) random row(s) from a table using SQLAlchemy? </p>
| 49 | 2008-09-13T19:58:02Z | 14,906,244 | <p>There is a simple way to pull a random row that IS database independent.
Just use .offset() . No need to pull all rows:</p>
<pre class="lang-python prettyprint-override"><code>import random
query = DBSession.query(Table)
rowCount = int(query.count())
randomRow = query.offset(int(rowCount*random.random())).first()
</code></pre>
<p>Where Table is your table (or you could put any query there).
If you want a few rows, then you can just run this multiple times, and make sure that each row is not identical to the previous.</p>
| 10 | 2013-02-16T02:19:56Z | [
"python",
"sql",
"database",
"sqlalchemy"
] |
Getting random row through SQLAlchemy | 60,805 | <p>How do I select a(or some) random row(s) from a table using SQLAlchemy? </p>
| 49 | 2008-09-13T19:58:02Z | 21,242,325 | <h2>this solution will select a single random row</h2>
<p>This solution requires that the primary key is named id, it should be if its not already: </p>
<pre><code>import random
max_model_id = YourModel.query.order_by(YourModel.id.desc())[0].id
random_id = random.randrange(0,max_model_id)
random_row = YourModel.query.get(random_id)
print random_row
</code></pre>
| -1 | 2014-01-20T19:18:12Z | [
"python",
"sql",
"database",
"sqlalchemy"
] |
Getting random row through SQLAlchemy | 60,805 | <p>How do I select a(or some) random row(s) from a table using SQLAlchemy? </p>
| 49 | 2008-09-13T19:58:02Z | 33,583,008 | <p>Here's four different variations, ordered from slowest to fastest. <code>timeit</code> results at the bottom:</p>
<pre><code>from sqlalchemy.sql import func
from sqlalchemy.orm import load_only
def simple_random():
return random.choice(model_name.query.all())
def load_only_random():
return random.choice(model_name.query.options(load_only('id')).all())
def order_by_random():
return model_name.query.order_by(func.random()).first()
def optimized_random():
return model_name.query.options(load_only('id')).offset(
func.floor(
func.random() *
db.session.query(func.count(model_name.id))
)
).limit(1).all()
</code></pre>
<p><code>timeit</code> results for 10,000 runs on my Macbook against a PostgreSQL table with 300 rows:</p>
<pre><code>simple_random():
90.09954111799925
load_only_random():
65.94714171699889
order_by_random():
23.17819356000109
optimized_random():
19.87806927999918
</code></pre>
<p>You can easily see that using <code>func.random()</code> is far faster than returning all results to Python's <code>random.choice()</code>. </p>
<p>Additionally, as the size of the table increases, the performance of <code>order_by_random()</code> will degrade significantly because an <code>ORDER BY</code> requires a full table scan versus the <code>COUNT</code> in <code>optimized_random()</code> can use an index.</p>
| 3 | 2015-11-07T12:55:09Z | [
"python",
"sql",
"database",
"sqlalchemy"
] |
How do you retrieve items from a dictionary in the order that they're inserted? | 60,848 | <p>Is it possible to retrieve items from a Python dictionary in the order that they were inserted?</p>
| 44 | 2008-09-13T20:38:05Z | 60,852 | <p>You can't do this with the base dict class -- it's ordered by hash. You could build your own dictionary that is really a list of key,value pairs or somesuch, which would be ordered.</p>
| 6 | 2008-09-13T20:39:58Z | [
"python",
"dictionary"
] |
How do you retrieve items from a dictionary in the order that they're inserted? | 60,848 | <p>Is it possible to retrieve items from a Python dictionary in the order that they were inserted?</p>
| 44 | 2008-09-13T20:38:05Z | 60,862 | <p>The standard python <code>dict</code> isn't able to do this.</p>
<p>There is a proposal (<a href="http://www.python.org/dev/peps/pep-0372/">PEP 372</a>) to add an "ordered dictionary" (that keeps track of the order of insertion) to the <code>collections</code> module in the standard library. It includes links to <a href="http://babel.edgewall.org/browser/trunk/babel/util.py?rev=374#L178">various</a> <a href="http://www.xs4all.nl/~anthon/Python/ordereddict/">implementations</a> <a href="http://code.djangoproject.com/browser/django/trunk/django/utils/datastructures.py?rev=7140#L53">of</a> <a href="http://pypi.python.org/pypi/StableDict/0.2">ordered</a> <a href="http://codespeak.net/svn/user/arigo/hack/pyfuse/OrderedDict.py">dictionaries</a> (see also these <a href="http://code.activestate.com/recipes/107747/">two</a> <a href="http://code.activestate.com/recipes/496761/">recipes</a> in the Python Cookbook).</p>
<p>You might want to stick with the reference implementation in the PEP if you want your code to be compatible with the "official" version (if the proposal is eventually accepted).</p>
<p>EDIT: The PEP was accepted and added in python 2.7 and 3.1. See <a href="http://docs.python.org/library/collections.html#ordereddict-objects">the docs</a>.</p>
| 44 | 2008-09-13T20:48:36Z | [
"python",
"dictionary"
] |
How do you retrieve items from a dictionary in the order that they're inserted? | 60,848 | <p>Is it possible to retrieve items from a Python dictionary in the order that they were inserted?</p>
| 44 | 2008-09-13T20:38:05Z | 61,031 | <p>The other answers are correct; it's not possible, but you could write this yourself. However, in case you're unsure how to actually implement something like this, here's a complete and working implementation that subclasses dict which I've just written and tested. (Note that the order of values passed to the constructor is undefined but will come before values passed later, and you could always just not allow ordered dicts to be initialized with values.)</p>
<pre><code>class ordered_dict(dict):
def __init__(self, *args, **kwargs):
dict.__init__(self, *args, **kwargs)
self._order = self.keys()
def __setitem__(self, key, value):
dict.__setitem__(self, key, value)
if key in self._order:
self._order.remove(key)
self._order.append(key)
def __delitem__(self, key):
dict.__delitem__(self, key)
self._order.remove(key)
def order(self):
return self._order[:]
def ordered_items(self):
return [(key,self[key]) for key in self._order]
od = ordered_dict()
od["hello"] = "world"
od["goodbye"] = "cruel world"
print od.order() # prints ['hello', 'goodbye']
del od["hello"]
od["monty"] = "python"
print od.order() # prints ['goodbye', 'monty']
od["hello"] = "kitty"
print od.order() # prints ['goodbye', 'monty', 'hello']
print od.ordered_items()
# prints [('goodbye','cruel world'), ('monty','python'), ('hello','kitty')]
</code></pre>
| 16 | 2008-09-14T00:58:30Z | [
"python",
"dictionary"
] |
How do you retrieve items from a dictionary in the order that they're inserted? | 60,848 | <p>Is it possible to retrieve items from a Python dictionary in the order that they were inserted?</p>
| 44 | 2008-09-13T20:38:05Z | 64,266 | <p>if you don't need the dict functionality, and only need to return tuples in the order you've inserted them, wouldn't a queue work better?</p>
| 0 | 2008-09-15T16:07:04Z | [
"python",
"dictionary"
] |
How do you retrieve items from a dictionary in the order that they're inserted? | 60,848 | <p>Is it possible to retrieve items from a Python dictionary in the order that they were inserted?</p>
| 44 | 2008-09-13T20:38:05Z | 65,326 | <p>I've used StableDict before with good success.</p>
<p><a href="http://pypi.python.org/pypi/StableDict/0.2" rel="nofollow">http://pypi.python.org/pypi/StableDict/0.2</a></p>
| 4 | 2008-09-15T18:15:46Z | [
"python",
"dictionary"
] |
How do you retrieve items from a dictionary in the order that they're inserted? | 60,848 | <p>Is it possible to retrieve items from a Python dictionary in the order that they were inserted?</p>
| 44 | 2008-09-13T20:38:05Z | 65,991 | <p>Or, just make the key a tuple with time.now() as the first field in the tuple.</p>
<p>Then you can retrieve the keys with dictname.keys(), sort, and voila!</p>
<p>Gerry</p>
| 5 | 2008-09-15T19:31:43Z | [
"python",
"dictionary"
] |
How do you retrieve items from a dictionary in the order that they're inserted? | 60,848 | <p>Is it possible to retrieve items from a Python dictionary in the order that they were inserted?</p>
| 44 | 2008-09-13T20:38:05Z | 103,211 | <p>It's not possible unless you store the keys in a separate list for referencing later.</p>
| 1 | 2008-09-19T15:56:35Z | [
"python",
"dictionary"
] |
How do you retrieve items from a dictionary in the order that they're inserted? | 60,848 | <p>Is it possible to retrieve items from a Python dictionary in the order that they were inserted?</p>
| 44 | 2008-09-13T20:38:05Z | 659,817 | <p>Or use any of the implementations for the <a href="http://www.python.org/dev/peps/pep-0372/" rel="nofollow">PEP-372</a> described <a href="http://www.python.org/dev/peps/pep-0372/#reference-implementation" rel="nofollow">here</a>, like the <a href="http://www.voidspace.org.uk/python/odict.html" rel="nofollow">odict module</a> from the <a href="http://www.voidspace.org.uk/python/pythonutils.html" rel="nofollow">pythonutils</a>.</p>
<p>I successfully used the pocoo.org implementation, it is as easy as replacing your</p>
<pre><code>my_dict={}
my_dict["foo"]="bar"
</code></pre>
<p>with</p>
<pre><code>my_dict=odict.odict()
my_dict["foo"]="bar"
</code></pre>
<p>and require just <a href="http://dev.pocoo.org/hg/sandbox/raw-file/tip/odict.py" rel="nofollow">this file</a></p>
| 2 | 2009-03-18T19:42:29Z | [
"python",
"dictionary"
] |
How do you retrieve items from a dictionary in the order that they're inserted? | 60,848 | <p>Is it possible to retrieve items from a Python dictionary in the order that they were inserted?</p>
| 44 | 2008-09-13T20:38:05Z | 24,028,185 | <p>What you can do is insert the values with a key representing the order inputted, and then call <code>sorted()</code> on the items.</p>
<pre><code>>>> obj = {}
>>> obj[1] = 'Bob'
>>> obj[2] = 'Sally'
>>> obj[3] = 'Joe'
>>> for k, v in sorted(obj.items()):
... print v
...
Bob
Sally
Joe
>>>
</code></pre>
| 0 | 2014-06-04T02:50:06Z | [
"python",
"dictionary"
] |
How do you retrieve items from a dictionary in the order that they're inserted? | 60,848 | <p>Is it possible to retrieve items from a Python dictionary in the order that they were inserted?</p>
| 44 | 2008-09-13T20:38:05Z | 26,637,218 | <p>Use OrderedDict(), available since version 2.7</p>
<p>Just a matter of curiosity: <br /></p>
<pre><code>a = {}
b = OrderedDict()
c = OredredDict()
a['key1'] = 'value1'
a['key2'] = 'value2'
b['key1'] = 'value1'
b['key2'] = 'value2'
c['key2'] = 'value2'
c['key1'] = 'value1'
print a == b #True
print a == c #True
print b == c #False
</code></pre>
| 5 | 2014-10-29T17:42:25Z | [
"python",
"dictionary"
] |
Where do the Python unit tests go? | 61,151 | <p>If you're writing a library, or an app, where do the unit test files go? </p>
<p>It's nice to separate the test files from the main app code, but it's awkward to put them into a "tests" subdirectory inside of the app root directory, because it makes it harder to import the modules that you'll be testing. </p>
<p>Is there a best practice here?</p>
| 286 | 2008-09-14T05:41:11Z | 61,168 | <p>I don't believe there is an established "best practice".</p>
<p>I put my tests in another directory outside of the app code. I then add the main app directory to sys.path (allowing you to import the modules from anywhere) in my test runner script (which does some other stuff as well) before running all the tests. This way I never have to remove the tests directory from the main code when I release it, saving me time and effort, if an ever so tiny amount.</p>
| 10 | 2008-09-14T06:46:03Z | [
"python",
"unit-testing",
"code-organization"
] |
Where do the Python unit tests go? | 61,151 | <p>If you're writing a library, or an app, where do the unit test files go? </p>
<p>It's nice to separate the test files from the main app code, but it's awkward to put them into a "tests" subdirectory inside of the app root directory, because it makes it harder to import the modules that you'll be testing. </p>
<p>Is there a best practice here?</p>
| 286 | 2008-09-14T05:41:11Z | 61,169 | <p>A common practice is to put the tests directory in the same parent directory as your module/package. So if your module was called foo.py your directory layout would look like:</p>
<pre><code>parent_dir/
foo.py
tests/
</code></pre>
<p>Of course there is no one way of doing it. You could also make a tests subdirectory and import the module using <a href="http://www.python.org/dev/peps/pep-0328/">absolute import</a>.</p>
<p>Wherever you put your tests, I would recommend you use <a href="http://nose.readthedocs.org/en/latest/">nose</a> to run them. <a href="http://nose.readthedocs.org/en/latest/">Nose</a> searches through your directories for tests. This way, you can put tests wherever they make the most sense organizationally.</p>
| 36 | 2008-09-14T06:46:14Z | [
"python",
"unit-testing",
"code-organization"
] |
Where do the Python unit tests go? | 61,151 | <p>If you're writing a library, or an app, where do the unit test files go? </p>
<p>It's nice to separate the test files from the main app code, but it's awkward to put them into a "tests" subdirectory inside of the app root directory, because it makes it harder to import the modules that you'll be testing. </p>
<p>Is there a best practice here?</p>
| 286 | 2008-09-14T05:41:11Z | 61,518 | <p>I've recently started to program in Python, so I've not really had chance to find out best practice yet.
But, I've written a module that goes and finds all the tests and runs them.</p>
<p>So, I have:</p>
<pre>
app/
appfile.py
test/
appfileTest.py
</pre>
<p>I'll have to see how it goes as I progress to larger projects.</p>
| -1 | 2008-09-14T18:02:12Z | [
"python",
"unit-testing",
"code-organization"
] |
Where do the Python unit tests go? | 61,151 | <p>If you're writing a library, or an app, where do the unit test files go? </p>
<p>It's nice to separate the test files from the main app code, but it's awkward to put them into a "tests" subdirectory inside of the app root directory, because it makes it harder to import the modules that you'll be testing. </p>
<p>Is there a best practice here?</p>
| 286 | 2008-09-14T05:41:11Z | 61,531 | <p>I use a <code>tests/</code> directory, and then import the main application modules using relative imports. So in MyApp/tests/foo.py, there might be:</p>
<pre><code>from .. import foo
</code></pre>
<p>to import the <code>MyApp.foo</code> module.</p>
| 13 | 2008-09-14T18:18:58Z | [
"python",
"unit-testing",
"code-organization"
] |
Where do the Python unit tests go? | 61,151 | <p>If you're writing a library, or an app, where do the unit test files go? </p>
<p>It's nice to separate the test files from the main app code, but it's awkward to put them into a "tests" subdirectory inside of the app root directory, because it makes it harder to import the modules that you'll be testing. </p>
<p>Is there a best practice here?</p>
| 286 | 2008-09-14T05:41:11Z | 61,820 | <p>In C#, I've generally separated the tests into a separate assembly.</p>
<p>In Python -- so far -- I've tended to either write doctests, where the test is in the docstring of a function, or put them in the <code>if __name__ == "__main__"</code> block at the bottom of the module.</p>
| 1 | 2008-09-15T03:09:29Z | [
"python",
"unit-testing",
"code-organization"
] |
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