Wrapper for docopt to allow ENV variables to override default arguments.

Example docopt pydoc:

"""
This is an example.

Usage:
    python my_example.py [options]

Options:
    -a --a-opt OPT  # An example long option with magic default
    -b OPT          # An example short option with normal default [default: buzz]
    --cee           # An example long switch option
    -d              # An example short switch option
"""

• The value of --a-opt will be taken using command line, or from the ENV variable $A_OPT. If neither is provided the value will be None
• The value of -b will be taken from the command line, from the ENV variable $B. If neither is provided the default value "buzz" will be used
• The value of --cee will be taken from the command line, or from the ENV variable $CEE. If neither is provided the flag will be set to False
• The value of -d will be taken from the command line, or from the ENV variable $D. If neither is provided the flag will be set to False

Notice the -a-opt option has been defined using a "magic" default. Because the default has been defined as the string describing the corresponding ENV variable, the value will not only be taken from the ENV at execution time, but will also print when the --help flag is provided.

So, given the following ENV configuration:

A_OPT='Hello, World!'

The help message of this script would be:

This is an example.

Usage:
    python my_example.py [options]

Options:
    -a --a-opt OPT  # An example long option with magic default [default: Hello, World!]
    -b OPT          # An example short option with normal default [default: buzz]
    --cee           # An example long switch option
    -d              # An example short switch option

Specifying a prefix value in the envopt() call will prefix ENV variables with this prefix right-padded with an underscore:

import os
from envopt import envopt

os.environ['MY_A_OPT'] = 'fizz'
os.environ['MY_B']     = '1'
os.environ['MY_CEE']   = 'buzz'
os.environ['MY_D']     = '1'

print envopt(__doc__, prefix='MY_')

{ '--a-opt' : 'fizz',
  '-b'      : 'buzz',
  '--cee'   : True,
  '-d'      : True }