# Adding Abseil (absl) flags quickstart

**WARNING** This module is deprecated. We no longer use it in new models and
your projects should not depend on it. We will remove this module when
all models using it are deprecated which may take time.

## Defining a flag
absl flag definitions are similar to argparse, although they are defined on a global namespace.

For instance, defining a string flag looks like this:
```$xslt
from absl import flags
flags.DEFINE_string(
    name="my_flag",
    default="a_sensible_default",
    help="Here is what this flag does."
)
```

All three arguments are required, but the default may be `None`. A common optional argument is
short_name for defining abbreviations. Certain `DEFINE_*` methods will have other required arguments.
For instance, `DEFINE_enum` requires the `enum_values` argument to be specified.

## Key Flags
absl has the concept of a key flag. Any flag defined in `__main__` is considered a key flag by
default. Key flags are displayed in `--help`, others only appear in `--helpful`. To
handle key flags that are defined outside the module in question, absl provides the
`flags.adopt_module_key_flags()` method. This adds the key flags of a different module to one's own
key flags. For example:
```$xslt
File: flag_source.py
---------------------------------------

from absl import flags
flags.DEFINE_string(name="my_flag", default="abc", help="a flag.")
```

```$xslt
File: my_module.py
---------------------------------------

from absl import app as absl_app
from absl import flags

import flag_source

flags.adopt_module_key_flags(flag_source)

def main(_):
  pass

absl_app.run(main, [__file__, "-h"]
```

when `my_module.py` is run it will show the help text for `my_flag`. Because not all flags defined
in a file are equally important, `official/utils/flags/core.py` (generally imported as flags_core)
provides an abstraction for easily handling key flag declaration through the
`register_key_flags_in_core()` function, which allows a module to make a single
`adopt_key_flags(flags_core)` call when using the util flag declaration functions.

## Validators
Often the constraints on a flag are complicated. absl provides the validator decorator to allow
one to mark a function as a flag validation function. Suppose we want users to provide a flag
that is a palindrome.

```$xslt
from absl import flags

flags.DEFINE_string(name="pal_flag", short_name="pf", default="", help="Give me a palindrome")

@flags.validator("pal_flag")
def _check_pal(provided_pal_flag):
  return provided_pal_flag == provided_pal_flag[::-1]

```

Validators take the form that returning True (truthy) passes, and all others 
(False, None, exception) fail.

## Testing
To test using absl, simply declare flags in the setupClass method of TensorFlow's TestCase.

```$xslt
from absl import flags
import tensorflow as tf

def define_flags():
  flags.DEFINE_string(name="test_flag", default="abc", help="an example flag")


class BaseTester(unittest.TestCase):

  @classmethod
  def setUpClass(cls):
    super(BaseTester, cls).setUpClass()
    define_flags()
    
  def test_trivial(self):
    flags_core.parse_flags([__file__, "test_flag", "def"])
    self.AssertEqual(flags.FLAGS.test_flag, "def")
    
```