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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 can automatically generate command line parameters from a function signature.

import plac

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)

The program above can now take parameters from the command line like so:

python example.py A 1000

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

usage: example.py [-h] model [iter] [debug]
example.py: error: the following arguments are required: model

In addition, the program can also generate a help message:

python example.py -h

Produces the following output:


usage: example.py [-h] model [iter] [debug]

A script for machine learning

positional arguments:
  model
  iter        [100]
  debug       [False]

options:
  -h, --help  show this help message and exit

When you need more control plac 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)

That will produce the following help:

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

Decorator reference

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

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

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

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

Issues

License

BSD License