Web Devs Quickstart

Installation

pip install python-fasthtml

A Minimal Application

A minimal FastHTML application looks something like this:

main.py

from fasthtml.common import *

app, rt = fast_app()

@rt("/")
def get():
    return Titled("FastHTML", P("Let's do this!"))

serve()

Line 1
We import what we need for rapid development! A carefully-curated set of FastHTML functions and other Python objects is brought into our global namespace for convenience.

Line 3
We instantiate a FastHTML app with the fast_app() utility function. This provides a number of really useful defaults that we’ll take advantage of later in the tutorial.

Line 5
We use the rt() decorator to tell FastHTML what to return when a user visits / in their browser.

Line 6
We connect this route to HTTP GET requests by defining a view function called get().

Line 7
A tree of Python function calls that return all the HTML required to write a properly formed web page. You’ll soon see the power of this approach.

Line 9
The serve() utility configures and runs FastHTML using a library called uvicorn.

Run the code:

python main.py

The terminal will look like this:

INFO:     Uvicorn running on http://0.0.0.0:5001 (Press CTRL+C to quit)
INFO:     Started reloader process [58058] using WatchFiles
INFO:     Started server process [58060]
INFO:     Waiting for application startup.
INFO:     Application startup complete.

Confirm FastHTML is running by opening your web browser to 127.0.0.1:5001. You should see something like the image below:

Note

While some linters and developers will complain about the wildcard import, it is by design here and perfectly safe. FastHTML is very deliberate about the objects it exports in fasthtml.common. If it bothers you, you can import the objects you need individually, though it will make the code more verbose and less readable.

If you want to learn more about how FastHTML handles imports, we cover that here.

A Minimal Charting Application

The Script function allows you to include JavaScript. You can use Python to generate parts of your JS or JSON like this:

import json
from fasthtml.common import * 

app, rt = fast_app(hdrs=(Script(src="https://cdn.plot.ly/plotly-2.32.0.min.js"),))

data = json.dumps({
    "data": [{"x": [1, 2, 3, 4],"type": "scatter"},
            {"x": [1, 2, 3, 4],"y": [16, 5, 11, 9],"type": "scatter"}],
    "title": "Plotly chart in FastHTML ",
    "description": "This is a demo dashboard",
    "type": "scatter"
})


@rt("/")
def get():
  return Titled("Chart Demo", Div(id="myDiv"),
    Script(f"var data = {data}; Plotly.newPlot('myDiv', data);"))

serve()

Debug Mode

When we can’t figure out a bug in FastHTML, we can run it in DEBUG mode. When an error is thrown, the error screen is displayed in the browser. This error setting should never be used in a deployed app.

from fasthtml.common import *

app, rt = fast_app(debug=True)

@rt("/")
def get():
    1/0
    return Titled("FastHTML Error!", P("Let's error!"))

serve()

Line 3
debug=True sets debug mode on.

Line 7
Python throws an error when it tries to divide an integer by zero.

Routing

FastHTML builds upon FastAPI’s friendly decorator pattern for specifying URLs, with extra features:

main.py

from fasthtml.common import * 

app, rt = fast_app()

@rt("/")
def get():
  return Titled("FastHTML", P("Let's do this!"))

@rt("/hello")
def get():
  return Titled("Hello, world!")

serve()

Line 5
The “/” URL on line 5 is the home of a project. This would be accessed at 127.0.0.1:5001.

Line 9
“/hello” URL on line 9 will be found by the project if the user visits 127.0.0.1:5001/hello.

Tip

It looks like get() is being defined twice, but that’s not the case. Each function decorated with rt is totally separate, and is injected into the router. We’re not calling them in the module’s namespace (locals()). Rather, we’re loading them into the routing mechanism using the rt decorator.

You can do more! Read on to learn what we can do to make parts of the URL dynamic.

Variables in URLs

You can add variable sections to a URL by marking them with {variable_name}. Your function then receives the {variable_name} as a keyword argument, but only if it is the correct type. Here’s an example:

main.py

from fasthtml.common import * 

app, rt = fast_app()

@rt("/{name}/{age}")
def get(name: str, age: int):
  return Titled(f"Hello {name.title()}, age {age}")

serve()

Line 5
We specify two variable names, name and age.

Line 6
We define two function arguments named identically to the variables. You will note that we specify the Python types to be passed.

Line 7
We use these functions in our project.

Try it out by going to this address: 127.0.0.1:5001/uma/5. You should get a page that says,

“Hello Uma, age 5”.

What happens if we enter incorrect data?

The 127.0.0.1:5001/uma/5 URL works because 5 is an integer. If we enter something that is not, such as 127.0.0.1:5001/uma/five, then FastHTML will return an error instead of a web page.

FastHTML URL routing supports more complex types

The two examples we provide here use Python’s built-in str and int types, but you can use your own types, including more complex ones such as those defined by libraries like attrs, pydantic, and even sqlmodel.

HTTP Methods

FastHTML matches function names to HTTP methods. So far the URL routes we’ve defined have been for HTTP GET methods, the most common method for web pages.

Form submissions often are sent as HTTP POST. When dealing with more dynamic web page designs, also known as Single Page Apps (SPA for short), the need can arise for other methods such as HTTP PUT and HTTP DELETE. The way FastHTML handles this is by changing the function name.

main.py

from fasthtml.common import * 

app, rt = fast_app()

@rt("/")  
def get():
  return Titled("HTTP GET", P("Handle GET"))

@rt("/")  
def post():
  return Titled("HTTP POST", P("Handle POST"))

serve()

Line 6
On line 6 because the get() function name is used, this will handle HTTP GETs going to the / URI.

Line 10
On line 10 because the post() function name is used, this will handle HTTP POSTs going to the / URI.

CSS Files and Inline Styles

Here we modify default headers to demonstrate how to use the Sakura CSS microframework instead of FastHTML’s default of Pico CSS.

main.py

from fasthtml.common import * 

app, rt = fast_app(
    pico=False,
    hdrs=(
        Link(rel='stylesheet', href='assets/normalize.min.css', type='text/css'),
        Link(rel='stylesheet', href='assets/sakura.css', type='text/css'),
        Style("p {color: red;}")
))

@app.get("/")
def home():
    return Titled("FastHTML",
        P("Let's do this!"),
    )

serve()

Line 4
By setting pico to False, FastHTML will not include pico.min.css.

Line 7
This will generate an HTML <link> tag for sourcing the css for Sakura.

Line 8
If you want an inline styles, the Style() function will put the result into the HTML.

Other Static Media File Locations

As you saw, Script and Link are specific to the most common static media use cases in web apps: including JavaScript, CSS, and images. But it also works with videos and other static media files. The default behavior is to look for these files in the root directory - typically we don’t do anything special to include them. We can change the default directory that is looked in for files by adding the static_path parameter to the fast_app function.

app, rt = fast_app(static_path='public')

FastHTML also allows us to define a route that uses FileResponse to serve the file at a specified path. This is useful for serving images, videos, and other media files from a different directory without having to change the paths of many files. So if we move the directory containing the media files, we only need to change the path in one place. In the example below, we call images from a directory called public.

@rt("/{fname:path}.{ext:static}")
async def get(fname:str, ext:str): 
    return FileResponse(f'public/{fname}.{ext}')

Rendering Markdown

from fasthtml.common import *

hdrs = (MarkdownJS(), HighlightJS(langs=['python', 'javascript', 'html', 'css']), )

app, rt = fast_app(hdrs=hdrs)

content = """
Here are some _markdown_ elements.

- This is a list item
- This is another list item
- And this is a third list item

**Fenced code blocks work here.**
"""

@rt('/')
def get(req):
    return Titled("Markdown rendering example", Div(content,cls="marked"))

serve()

Code highlighting

Here’s how to highlight code without any markdown configuration.

from fasthtml.common import *

# Add the HighlightJS built-in header
hdrs = (HighlightJS(langs=['python', 'javascript', 'html', 'css']),)

app, rt = fast_app(hdrs=hdrs)

code_example = """
import datetime
import time

for i in range(10):
    print(f"{datetime.datetime.now()}")
    time.sleep(1)
"""

@rt('/')
def get(req):
    return Titled("Markdown rendering example",
        Div(
            # The code example needs to be surrounded by
            # Pre & Code elements
            Pre(Code(code_example))
    ))

serve()

Defining new `ft` components

We can build our own ft components and combine them with other components. The simplest method is defining them as a function.

from fasthtml.common import *

def hero(title, statement):
    return Div(H1(title),P(statement), cls="hero")

# usage example
Main(
    hero("Hello World", "This is a hero statement")
)

<main>  <div class="hero">
    <h1>Hello World</h1>
    <p>This is a hero statement</p>
  </div>
</main>

Pass through components

For when we need to define a new component that allows zero-to-many components to be nested within them, we lean on Python’s *args and **kwargs mechanism. Useful for creating page layout controls.

def layout(*args, **kwargs):
    """Dashboard layout for all our dashboard views"""
    return Main(
        H1("Dashboard"),
        Div(*args, **kwargs),
        cls="dashboard",
    )

# usage example
layout(
    Ul(*[Li(o) for o in range(3)]),
    P("Some content", cls="description"),
)

<main class="dashboard">  <h1>Dashboard</h1>
  <div>
    <ul>
      <li>0</li>
      <li>1</li>
      <li>2</li>
    </ul>
    <p class="description">Some content</p>
  </div>
</main>

Dataclasses as ft components

While functions are easy to read, for more complex components some might find it easier to use a dataclass.

from dataclasses import dataclass

@dataclass
class Hero:
    title: str
    statement: str
    
    def __ft__(self):
        """ The __ft__ method renders the dataclass at runtime."""
        return Div(H1(self.title),P(self.statement), cls="hero")
    
# usage example
Main(
    Hero("Hello World", "This is a hero statement")
)

<main>  <div class="hero">
    <h1>Hello World</h1>
    <p>This is a hero statement</p>
  </div>
</main>

Testing views in notebooks

Because of the ASGI event loop it is currently impossible to run FastHTML inside a notebook. However, we can still test the output of our views. To do this, we leverage Starlette, an ASGI toolkit that FastHTML uses.

# First we instantiate our app, in this case we remove the
# default headers to reduce the size of the output.
app, rt = fast_app(default_hdrs=False)

# Setting up the Starlette test client
from starlette.testclient import TestClient
client = TestClient(app)

# Usage example
@rt("/")
def get():
    return Titled("FastHTML is awesome", 
        P("The fastest way to create web apps in Python"))

print(client.get("/").text)

 <!doctype html>
 <html>
   <head>
<title>FastHTML is awesome</title>   </head>
   <body>
<main class="container">       <h1>FastHTML is awesome</h1>
       <p>The fastest way to create web apps in Python</p>
</main>   </body>
 </html>

Forms

To validate data coming from users, first define a dataclass representing the data you want to check. Here’s an example representing a signup form.

from dataclasses import dataclass

@dataclass
class Profile: email:str; phone:str; age:int

Create an FT component representing an empty version of that form. Don’t pass in any value to fill the form, that gets handled later.

profile_form = Form(method="post", action="/profile")(
        Fieldset(
            Label('Email', Input(name="email")),
            Label("Phone", Input(name="phone")),
            Label("Age", Input(name="age")),
        ),
        Button("Save", type="submit"),
    )
profile_form

<form enctype="multipart/form-data" method="post" action="/profile"><fieldset><label>Email      <input name="email">
</label><label>Phone      <input name="phone">
</label><label>Age      <input name="age">
</label></fieldset><button type="submit">Save</button></form>

Once the dataclass and form function are completed, we can add data to the form. To do that, instantiate the profile dataclass:

profile = Profile(email='[email protected]', phone='123456789', age=5)
profile

Profile(email='[email protected]', phone='123456789', age=5)

Then add that data to the profile_form using FastHTML’s fill_form class:

fill_form(profile_form, profile)

<form enctype="multipart/form-data" method="post" action="/profile"><fieldset><label>Email      <input name="email" value="[email protected]">
</label><label>Phone      <input name="phone" value="123456789">
</label><label>Age      <input name="age" value="5">
</label></fieldset><button type="submit">Save</button></form>

Forms with views

The usefulness of FastHTML forms becomes more apparent when they are combined with FastHTML views. We’ll show how this works by using the test client from above. First, let’s create a SQlite database:

db = database("profiles.db")
profiles = db.create(Profile, pk="email")

Now we insert a record into the database:

profiles.insert(profile)

Profile(email='[email protected]', phone='123456789', age=5)

And we can then demonstrate in the code that form is filled and displayed to the user.

@rt("/profile/{email}")
def profile(email:str):
    profile = profiles[email]
    filled_profile_form = fill_form(profile_form, profile)
    return Titled(f'Profile for {profile.email}', filled_profile_form)

print(client.get(f"/profile/[email protected]").text)

Line 3
Fetch the profile using the profile table’s email primary key

Line 4
Fill the form for display.

 <!doctype html>
 <html>
   <head>
<title>Profile for [email protected]</title>   </head>
   <body>
<main class="container">       <h1>Profile for [email protected]</h1>
<form enctype="multipart/form-data" method="post" action="/profile"><fieldset><label>Email             <input name="email" value="[email protected]">
</label><label>Phone             <input name="phone" value="123456789">
</label><label>Age             <input name="age" value="5">
</label></fieldset><button type="submit">Save</button></form></main>   </body>
 </html>

And now let’s demonstrate making a change to the data.

@rt("/profile")
def post(profile: Profile):
    profiles.update(profile)
    return RedirectResponse(url=f"/profile/{profile.email}")

new_data = dict(email='[email protected]', phone='7654321', age=25)
print(client.post("/profile", data=new_data).text)

Line 2
We use the Profile dataclass definition to set the type for the incoming profile content. This validates the field types for the incoming data

Line 3
Taking our validated data, we updated the profiles table

Line 4
We redirect the user back to their profile view

Line 7
The display is of the profile form view showing the changes in data.

 <!doctype html>
 <html>
   <head>
<title>Profile for [email protected]</title>   </head>
   <body>
<main class="container">       <h1>Profile for [email protected]</h1>
<form enctype="multipart/form-data" method="post" action="/profile"><fieldset><label>Email             <input name="email" value="[email protected]">
</label><label>Phone             <input name="phone" value="7654321">
</label><label>Age             <input name="age" value="25">
</label></fieldset><button type="submit">Save</button></form></main>   </body>
 </html>

Strings and conversion order

The general rules for rendering are: - __ft__ method will be called (for default components like P, H2, etc. or if you define your own components) - If you pass a string, it will be escaped - On other python objects, str() will be called

As a consequence, if you want to include plain HTML tags directly into e.g. a Div() they will get escaped by default (as a security measure to avoid code injections). This can be avoided by using NotStr(), a convenient way to reuse python code that returns already HTML. If you use pandas, you can use pandas.DataFrame.to_html() to get a nice table. To include the output a FastHTML, wrap it in NotStr(), like Div(NotStr(df.to_html())).

Above we saw how a dataclass behaves with the __ft__ method defined. On a plain dataclass, str() will be called (but not escaped).

from dataclasses import dataclass

@dataclass
class Hero:
    title: str
    statement: str
        
# rendering the dataclass with the default method
Main(
    Hero("<h1>Hello World</h1>", "This is a hero statement")
)

<main>Hero(title='<h1>Hello World</h1>', statement='This is a hero statement')</main>

# This will display the HTML as text on your page
Div("Let's include some HTML here: <div>Some HTML</div>")

<div>Let&#x27;s include some HTML here: &lt;div&gt;Some HTML&lt;/div&gt;</div>

# Keep the string untouched, will be rendered on the page
Div(NotStr("<div><h1>Some HTML</h1></div>"))

<div><div><h1>Some HTML</h1></div></div>

Custom exception handlers

FastHTML allows customization of exception handlers, but does so gracefully. What this means is by default it includes all the <html> tags needed to display attractive content. Try it out!

from fasthtml.common import *

def not_found(req, exc): return Titled("404: I don't exist!")

exception_handlers = {404: not_found}

app, rt = fast_app(exception_handlers=exception_handlers)

@rt('/')
def get():
    return (Titled("Home page", P(A(href="/oops")("Click to generate 404 error"))))

serve()

We can also use lambda to make things more terse:

from fasthtml.common import *

exception_handlers={
    404: lambda req, exc: Titled("404: I don't exist!"),
    418: lambda req, exc: Titled("418: I'm a teapot!")
}

app, rt = fast_app(exception_handlers=exception_handlers)

@rt('/')
def get():
    return (Titled("Home page", P(A(href="/oops")("Click to generate 404 error"))))

serve()

Cookies

We can set cookies using the cookie() function. In our example, we’ll create a timestamp cookie.

from datetime import datetime
from IPython.display import HTML

@rt("/settimestamp")
def get(req):
    now = datetime.now()
    return P(f'Set to {now}'), cookie('now', datetime.now())

HTML(client.get('/settimestamp').text)

FastHTML page

Set to 2024-09-26 15:33:48.141869

Now let’s get it back using the same name for our parameter as the cookie name.

@rt('/gettimestamp')
def get(now:parsed_date): return f'Cookie was set at time {now.time()}'

client.get('/gettimestamp').text

'Cookie was set at time 15:33:48.141903'

Sessions

For convenience and security, FastHTML has a mechanism for storing small amounts of data in the user’s browser. We can do this by adding a session argument to routes. FastHTML sessions are Python dictionaries, and we can leverage to our benefit. The example below shows how to concisely set and get sessions.

@rt('/adder/{num}')
def get(session, num: int):
    session.setdefault('sum', 0)
    session['sum'] = session.get('sum') + num
    return Response(f'The sum is {session["sum"]}.')

Toasts (also known as Messages)

Toasts, sometimes called “Messages” are small notifications usually in colored boxes used to notify users that something has happened. Toasts can be of four types:

info
success
warning
error

Examples toasts might include:

“Payment accepted”
“Data submitted”
“Request approved”

Toasts require the use of the setup_toasts() function plus every view needs these two features:

The session argument
Must return FT components

setup_toasts(app)

@rt('/toasting')
def get(session):
    # Normally one toast is enough, this allows us to see
    # different toast types in action.
    add_toast(session, f"Toast is being cooked", "info")
    add_toast(session, f"Toast is ready", "success")
    add_toast(session, f"Toast is getting a bit crispy", "warning")
    add_toast(session, f"Toast is burning!", "error")
    return Titled("I like toast")

Line 1
setup_toasts is a helper function that adds toast dependencies. Usually this would be declared right after fast_app()

Line 4
Toasts require sessions

Line 11
Views with Toasts must return FT or FtResponse components.

💡 setup_toasts takes a duration input that allows you to specify how long a toast will be visible before disappearing. For example setup_toasts(duration=5) sets the toasts duration to 5 seconds. By default toasts disappear after 10 seconds.

⚠️ Toasts don’t work with SPA like navigation that replaces the entire body such as this navigation trigger A('About', hx_get="/about", hx_swap="outerHTML", hx_push_url="true", hx_target="body"). As an alternative, wrap the content of your route in an element containing an id and set this id as the target for your navigation trigger (i.e. hx_target='#container_id').

Authentication and authorization

In FastHTML the tasks of authentication and authorization are handled with Beforeware. Beforeware are functions that run before the route handler is called. They are useful for global tasks like ensuring users are authenticated or have permissions to access a view.

First, we write a function that accepts a request and session arguments:

# Status code 303 is a redirect that can change POST to GET,
# so it's appropriate for a login page.
login_redir = RedirectResponse('/login', status_code=303)

def user_auth_before(req, sess):
    # The `auth` key in the request scope is automatically provided
    # to any handler which requests it, and can not be injected
    # by the user using query params, cookies, etc, so it should
    # be secure to use.    
    auth = req.scope['auth'] = sess.get('auth', None)
    # If the session key is not there, it redirects to the login page.
    if not auth: return login_redir

Now we pass our user_auth_before function as the first argument into a Beforeware class. We also pass a list of regular expressions to the skip argument, designed to allow users to still get to the home and login pages.

beforeware = Beforeware(
    user_auth_before,
    skip=[r'/favicon\.ico', r'/static/.*', r'.*\.css', r'.*\.js', '/login', '/']
)

app, rt = fast_app(before=beforeware)

Server-sent events (SSE)

With server-sent events, it’s possible for a server to send new data to a web page at any time, by pushing messages to the web page. Unlike WebSockets, SSE can only go in one direction: server to client. SSE is also part of the HTTP specification unlike WebSockets which uses its own specification.

FastHTML introduces several tools for working with SSE which are covered in the example below. While concise, there’s a lot going on in this function so we’ve annotated it quite a bit.

import random
from asyncio import sleep
from fasthtml.common import *

hdrs=(Script(src="https://unpkg.com/[email protected]/sse.js"),)
app,rt = fast_app(hdrs=hdrs)

@rt
def index():
    return Titled("SSE Random Number Generator",
        P("Generate pairs of random numbers, as the list grows scroll downwards."),
        Div(hx_ext="sse",
            sse_connect="/number-stream",
            hx_swap="beforeend show:bottom",
            sse_swap="message"))

shutdown_event = signal_shutdown()

async def number_generator():
    while not shutdown_event.is_set():
        data = Article(random.randint(1, 100))
        yield sse_message(data)
        await sleep(1)

@rt("/number-stream")
async def get(): return EventStream(number_generator())

Line 5
Import the HTMX SSE extension

Line 12
Tell HTMX to load the SSE extension

Line 13
Look at the /number-stream endpoint for SSE content

Line 14
When new items come in from the SSE endpoint, add them at the end of the current content within the div. If they go beyond the screen, scroll downwards

Line 15
Specify the name of the event. FastHTML’s default event name is “message”. Only change if you have more than one call to SSE endpoints within a view

Line 17
Set up the asyncio event loop

Line 19
Don’t forget to make this an async function!

Line 20
Iterate through the asyncio event loop

Line 22
We yield the data. Data ideally should be comprised of FT components as that plugs nicely into HTMX in the browser

Line 26
The endpoint view needs to be an async function that returns a EventStream

Websockets

With websockets we can have bi-directional communications between a browser and client. Websockets are useful for things like chat and certain types of games. While websockets can be used for single direction messages from the server (i.e. telling users that a process is finished), that task is arguably better suited for SSE.

FastHTML provides useful tools for adding websockets to your pages.

from fasthtml.common import *
from asyncio import sleep

app, rt = fast_app(exts='ws')

def mk_inp(): return Input(id='msg', autofocus=True)

@rt('/')
async def get(request):
    cts = Div(
        Div(id='notifications'),
        Form(mk_inp(), id='form', ws_send=True),
        hx_ext='ws', ws_connect='/ws')
    return Titled('Websocket Test', cts)

async def on_connect(send):
    print('Connected!')
    await send(Div('Hello, you have connected', id="notifications"))

async def on_disconnect(ws):
    print('Disconnected!')

@app.ws('/ws', conn=on_connect, disconn=on_disconnect)
async def ws(msg:str, send):
    await send(Div('Hello ' + msg, id="notifications"))
    await sleep(2)
    return Div('Goodbye ' + msg, id="notifications"), mk_inp()

Line 4
To use websockets in FastHTML, you must instantiate the app with exts set to ‘ws’

Line 6
As we want to use websockets to reset the form, we define the mk_input function that can be called from multiple locations

Line 12
We create the form and mark it with the ws_send attribute, which is documented here in the HTMX websocket specification. This tells HTMX to send a message to the nearest websocket based on the trigger for the form element, which for forms is pressing the enter key, an action considered to be a form submission

Line 13
This is where the HTMX extension is loaded (hx_ext='ws') and the nearest websocket is defined (ws_connect='/ws')

Line 16
When a websocket first connects we can optionally have it call a function that accepts a send argument. The send argument will push a message to the browser.

Line 18
Here we use the send function that was passed into the on_connect function to send a Div with an id of notifications that HTMX assigns to the element in the page that already has an id of notifications

Line 20
When a websocket disconnects we can call a function which takes no arguments. Typically the role of this function is to notify the server to take an action. In this case, we print a simple message to the console

Line 23
We use the app.ws decorator to mark that /ws is the route for our websocket. We also pass in the two optional conn and disconn parameters to this decorator. As a fun experiment, remove the conn and disconn arguments and see what happens

Line 24
Define the ws function as async. This is necessary for ASGI to be able to serve websockets. The function accepts two arguments, a msg that is user input from the browser, and a send function for pushing data back to the browser

Line 25
The send function is used here to send HTML back to the page. As the HTML has an id of notifications, HTMX will overwrite what is already on the page with the same ID

Line 27
The websocket function can also be used to return a value. In this case, it is a tuple of two HTML elements. HTMX will take the elements and replace them where appropriate. As both have id specified (notifications and msg respectively), they will replace their predecessor on the page.

File Uploads

A common task in web development is uploading files. The examples below are for uploading files to the hosting server, with information about the uploaded file presented to the user.

File uploads in production can be dangerous

File uploads can be the target of abuse, accidental or intentional. That means users may attempt to upload files that are too large or present a security risk. This is especially of concern for public facing apps. File upload security is outside the scope of this tutorial, for now we suggest reading the OWASP File Upload Cheat Sheet.

Single File Uploads

from fasthtml.common import *
from pathlib import Path

app, rt = fast_app()

upload_dir = Path("filez")
upload_dir.mkdir(exist_ok=True)

@rt('/')
def get():
    return Titled("File Upload Demo",
        Article(
            Form(hx_post=upload, hx_target="#result-one")(
                Input(type="file", name="file"),
                Button("Upload", type="submit", cls='secondary'),
            ),
            Div(id="result-one")
        )
    )

def FileMetaDataCard(file):
    return Article(
        Header(H3(file.filename)),
        Ul(
            Li('Size: ', file.size),            
            Li('Content Type: ', file.content_type),
            Li('Headers: ', file.headers),
        )
    )    

@rt
async def upload(file: UploadFile):
    card = FileMetaDataCard(file)
    filebuffer = await file.read()
    (upload_dir / file.filename).write_bytes(filebuffer)
    return card

serve()

Line 13
Every form rendered with the Form FT component defaults to enctype="multipart/form-data"

Line 14
Don’t forget to set the Input FT Component’s type to file

Line 32
The upload view should receive a Starlette UploadFile type. You can add other form variables

Line 33
We can access the metadata of the card (filename, size, content_type, headers), a quick and safe process. We set that to the card variable

Line 34
In order to access the contents contained within a file we use the await method to read() it. As files may be quite large or contain bad data, this is a seperate step from accessing metadata

Line 35
This step shows how to use Python’s built-in pathlib.Path library to write the file to disk.

Multiple File Uploads

from fasthtml.common import *
from pathlib import Path

app, rt = fast_app()

upload_dir = Path("filez")
upload_dir.mkdir(exist_ok=True)

@rt('/')
def get():
    return Titled("Multiple File Upload Demo",
        Article(
            Form(hx_post=upload_many, hx_target="#result-many")(
                Input(type="file", name="files", multiple=True),
                Button("Upload", type="submit", cls='secondary'),
            ),
            Div(id="result-many")
        )
    )

def FileMetaDataCard(file):
    return Article(
        Header(H3(file.filename)),
        Ul(
            Li('Size: ', file.size),            
            Li('Content Type: ', file.content_type),
            Li('Headers: ', file.headers),
        )
    )    

@rt
async def upload_many(files: list[UploadFile]):
    cards = []
    for file in files:
        cards.append(FileMetaDataCard(file))
        filebuffer = await file.read()
        (upload_dir / file.filename).write_bytes(filebuffer)
    return cards

serve()

Line 13
Every form rendered with the Form FT component defaults to enctype="multipart/form-data"

Line 14
Don’t forget to set the Input FT Component’s type to file and assign the multiple attribute to True

Line 32
The upload view should receive a list containing the Starlette UploadFile type. You can add other form variables

Line 34
Iterate through the files

Line 35
We can access the metadata of the card (filename, size, content_type, headers), a quick and safe process. We add that to the cards variable

Line 36
In order to access the contents contained within a file we use the await method to read() it. As files may be quite large or contain bad data, this is a seperate step from accessing metadata

Line 37
This step shows how to use Python’s built-in pathlib.Path library to write the file to disk.

Web Devs Quickstart

Installation

A Minimal Application

A Minimal Charting Application

Debug Mode

Routing

Variables in URLs

What happens if we enter incorrect data?

HTTP Methods

CSS Files and Inline Styles

Other Static Media File Locations

Rendering Markdown

Code highlighting

Defining new ft components

Pass through components

Dataclasses as ft components

Testing views in notebooks

Forms

Forms with views

Strings and conversion order

Custom exception handlers

Cookies

Sessions

Toasts (also known as Messages)

Authentication and authorization

Server-sent events (SSE)

Websockets

File Uploads

Single File Uploads

Multiple File Uploads

Defining new `ft` components