Plac: parsing the command line the easy way

plac is a Python package that can generate command line parameters from function signatures.

plac works on Python 2.6 through all versions of Python 3.

plac has no dependencies beyond modules already present in the Python standard library.

plac implements most of its functionality in a single file that may be included in your source code.

Quickstart

plac automatically generates the command line parameters from the function signature.

It offers three decorators to describe positional, option and flag type parameters:

import plac

# Add decorators to the function
@plac.pos('model', help="model name", choices=['A', 'B', 'C'])
@plac.opt('iter', help="iterations", type=int)
@plac.flg('debug', help="debug mode")
def main(model, iter=100, debug=False):
    """
    A script for machine learning
    """
    print (model, iter, debug)

if __name__ == '__main__':
    # Execute function via plac.call()
    plac.call(main)

And that's it! The program can now take parameters from the command line like so:

python example.py -d -i 1000 B 

Running the script with python example.py -h will give you the following help message: :

usage: example.py [-h] [-i 100] [-d] {A,B,C}

A script for machine learning

positional arguments:
  {A,B,C}             model name

options:
  -h, --help          show this help message and exit
  -i 100, --iter 100  iterations
  -d, --debug         debug mode

Running the script with no parameters python example.py would print:

usage: example.py [-h] [-i 100] [-d] {A,B,C}
example.py: error: the following arguments are required: model

Decorator reference

To use plac all you need to know are the following three decorators:

• @plac.pos - for positional parameters model
• @plac.opt - for key value options --iter 100
• @plac.flg - for flags --debug

that have the following signatures:

# Positional parameters.
pos(arg, help=None, type=None, choices=None, metavar=None):

# Option parameters.
opt(arg, help=None, type=None, abbrev=None, choices=None, metavar=None):

# Flag parameters.
flg(arg, help=None, abbrev=None):

Zero dependencies ... not even plac :-)

Notably, the main functionality of plac is implemented in a single Python module called plac_core.py that, if necessary, may be included and distributed with your source code thus reducing external dependencies in your code.

Copy plac_core.py to your package then use it like so:

from mypackage import plac_core as plac

Avoiding name clashes

Python syntax, or your variable naming may impose constraints on what words may be used as parameters. To circumvent that limitation append a trailing underscore to the name. plac will strip that underscore from the command line parameter name:

import plac

@plac.flg('list_')   # avoid clash with builtin
@plac.flg('yield_')  # avoid clash with keyword
@plac.opt('sys_')    # avoid clash with a very common name
def main(list_, yield_=False, sys_=100):
    print(list_)
    print(yield_)
    print(sys_)

if __name__ == '__main__':
    plac.call(main)

produces the usage:

usage: example13.py [-h] [-l] [-y] [-s 100]

optional arguments:
  -h, --help         show this help message and exit
  -l, --list
  -y, --yield        [False]
  -s 100, --sys 100  [100]

Variable arguments

plac may accept multiple positional arguments and even additional key=value pairs:

import plac

@plac.pos('args', help="words")
@plac.opt('kwds', help="key=value", )
def main(*args, **kwds):
    print(args)
    print(kwds)

if __name__ == '__main__':
    plac.call(main)

the usage will be:

usage: example15.py [-h] [args ...] [kwds ...]

positional arguments:
  args        words
  kwds        key=value

optional arguments:
  -h, --help  show this help message and exit

when running it as:

python example15.py A B x=10 y=20

the program prints:

('A', 'B')
{'x': '10', 'y': '20'}

Installation

pip install plac

Testing

Run

python doc/test_plac.py

You will see several apparent errors, but this is right, since the tests are checking for several error conditions. The important thing is that you get a line like

Executed XX tests OK

Code

• https://github.com/ialbert/plac

Author: Michele Simionato, michele.simionato@gmail.com

Maintainer: Istvan Albert, istvan.albert@gmail.com

Issues

• https://github.com/ialbert/plac/issues

License

BSD License