Pythonic option parser, that will make you smile
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docopt – pythonic option parser, that will make you smile

pip install docopt

Isn't it awesome how optparse and argparse generate help and usage-messages based on your code?!

Hell no! You know what's awesome? It's when the option parser is generated based on the help and usage-message that you write in a docstring! This way you don't need to write this stupid repeatable parser-code, and instead can write a beautiful usage-message (the way you want it!), which adds readability to your code.

So instead of writing shit like this (typical example):

from optparse import OptionParser

def process_options():
    parser = OptionParser(usage=" [options] arguments")
    parser.add_option('-v', '--verbose', action='store_true',
                      help="print status messages")
    parser.add_option('-q', '--quiet', action='store_true',
                      help="report only file names")
    parser.add_option('-r', '--repeat', action='store_true',
                      help="show all occurrences of the same error")
    parser.add_option('--exclude', metavar='patterns',
                      help="exclude files or directories which match these "
                        "comma separated patterns [default: %s]" %
    parser.add_option('--filename', metavar='patterns', default='*.py',
                      help="when parsing directories, only check filenames "
                        "matching these comma separated patterns "
                        "[default: *.py]")
    parser.add_option('--select', metavar='errors',
                      help="select errors and warnings (e.g. E,W6)")
    parser.add_option('--ignore', metavar='errors',
                      help="skip errors and warnings (e.g. E4,W)")
    parser.add_option('--show-source', action='store_true',
                      help="show source code for each error")
    options, arguments = parser.parse_args()
    return options, arguments

def main(options, arguments):
    pass  # ...

if __name__ == '__main__':
    options, arguments = process_options()
    main(options, arguments)

You can write an awesome, readable, clean, pythonic code like that:

"""Usage: [options] arguments

  -h --help            show this help message and exit
  -v --verbose         print status messages
  -q --quiet           report only file names
  -r --repeat          show all occurrences of the same error
  --exclude=patterns   exclude files or directories which match these comma
                       separated patterns [default: .svn,CVS,.bzr,.hg,.git]
  --filename=patterns  when parsing directories, only check filenames matching
                       these comma separated patterns [default: *.py]
  --select=errors      select errors and warnings (e.g. E,W6)
  --ignore=errors      skip errors and warnings (e.g. E4,W)
  --show-source        show source code for each error

from docopt import docopt

def main(options, arguments):
    pass  # ...

if __name__ == '__main__':
    # parse options based on docstring above
    options, arguments = docopt(__doc__)
    main(options, arguments)

Fuck yeah! The option parser is generated based on docstring above, that you pass to the docopt funciton.

Also, the practice of putting usage-message in module's docstring is endorsed by pep257:

The docstring of a script (a stand-alone program) should be usable as its "usage" message, printed when the script is invoked with incorrect or missing arguments (or perhaps with a "-h" option, for "help"). Such a docstring should document the script's function and command line syntax, environment variables, and files. Usage messages can be fairly elaborate (several screens full) and should be sufficient for a new user to use the command properly, as well as a complete quick reference to all options and arguments for the sophisticated user.

By the way, docopt is tested with Python 2.6, 2.7 and 3.2.

API from docopt import docopt

options, arguments = docopt(doc[, args=sys.argv[1:], help=True, version=None])

docopt takes 1 required and 3 optional arguments:

  • doc should be a module docstring (__doc__) or some other string that describes options in a human-readable format, that will be parsed to create the option parser. The simple rules of how to write such a docstring (in order to generate option parser from it successfully) are given in the next section. Here is a quick example of such a string:

    """Usage: [options]
    -h --help     Show this.
    -v --verbose  Print more text.
    --quiet       Print less text.
    -o FILE       Specify output file [default: ./test.txt]."""
  • args is an optional argument; by default it is supplied with options and arguments passed to your program (sys.argv[1:]). In case you want to supply something else, it should be in the format similar to sys.argv, i.e. a list of strings, such as ['--verbose', '-o', 'hai.txt'].

  • help, by default True, specifies whether the parser should automatically print the usage-message (supplied as doc) in case -h or --help options are encountered. After showing the usage-message, the program will terminate. If you want to handle -h or --help options manually (as all other options), set help=False.

  • version, by default None, is an optional argument that specifies the version of your program. If supplied, then, if the parser encounters --version option, it will print the supplied version and terminate. version could be any printable object, but most likely a string, e.g. "2.1.0rc1".

Note, when docopt is set to automatically handle -h, --help and --version options, you still need to mention them in the options description (doc) for your users to know about them.

The return value is a tuple options, arguments, where:

  • options is an object with instance variables corresponding to each option. It can be pretty-printed for debugging (try Names of instance variables will be based on option names, so that characters that are not allowed in an instance variable name (such as dash -) will be substituted with underscore _. E.g. option --print-out will be presented as options.print_out, and option -v, --verbose will be presented as options.verbose, giving precedence to a longer variant.

  • arguments is a list of non-option arguments.

Docstring format for your usage-message

The main idea behind docopt is that a good usage-message (that describes options and defaults unambiguously) is enough to generate an option parser.

Here are the simple rules (that you probably already follow) for your usage-message to be parsable:

  • Every line that starts with - or -- (not counting spaces) is treated as an option description, e.g.:

      --verbose   # GOOD
      -o FILE     # GOOD
    Other: --bad  # BAD, line does not start with dash "-"
  • To specify that an option has an argument, put a word describing that argument after space (or equals = sign) as shown below. You can use comma if you want to separate options. In the example below both lines are valid, however you are recommended to stick to a single style.

    -o FILE --output=FILE       # without comma, with "=" sign
    -i <file>, --input <file>   # with comma, wihtout "=" sing
  • Use two spaces to separate options with their informal description.

    --verbose More text.   # BAD, will be treated as if verbose option had
                           # an argument "More", so use 2 spaces instead
    -q        Quit.        # GOOD
    -o FILE   Output file. # GOOD
    --stdout  Use stdout.  # GOOD, 2 spaces
  • If you want to set a default value for an option with an argument, put it into the option description, in form [default: <your-default-value>]. To be precise, it should match the following regular expression: "\[default: (.*)\]". The parser will try to interprete the default value as Python literal (using ast.literal_eval), and if it can't, it will be interpreted as a string, e.g.:

    -i INSTANCE      Instance of something [default: 1]  # will be int
    --coefficient=K  The K coefficient [default: 2.95]   # will be float
    --output=FILE    Output file [default: "test.txt"]   # will be str
    --directory=DIR  Some directory [default: ./]        # will be str "./"

Note, that docopt also tries to interprete passed arguments of options as Python literals, or else as strings, so in most cases you don't need to convert types.

Something missing?

Missing a feature from your current option-parser? Together we can make docopt better, so send a patch or pull-request.