Add PEP 389: argparse - new command line parsing module.
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PEP: 389
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Title: argparse - new command line parsing module
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Version: $Revision$
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Last-Modified: $Date$
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Author: Steven Bethard <steven.bethard@gmail.com>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 25-Sep-2009
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Python-Version: 2.7 and 3.2
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Post-History:
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Abstract
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========
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This PEP proposes inclusion of the argparse [1]_ module in the Python
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standard library in Python 2.7 and 3.2.
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Motivation
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==========
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The argparse module is a command line parsing library which provides
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more functionality than the existing command line parsing modules in
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the standard library, getopt [2]_ and optparse [3]_. It includes
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support for positional arguments (not just options), subcommands,
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required options, options syntaxes like "/f" and "+rgb", zero-or-more
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and one-or-more style arguments, and many other features the other
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two lack.
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The argparse module is also already a popular third-party replacement
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for these modules. It is used in projects like IPython (the Scipy
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Python shell) [4]_, is included in Debian testing and unstable [5]_,
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and since 2007 has had various requests for its inclusion in the
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standard library [6]_ [7]_ [8]_. This popularity suggests it may be
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a valuable addition to the Python libraries.
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Why aren't getopt and optparse enough?
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======================================
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One argument against adding argparse is that thare are "already two
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different option parsing modules in the standard library" [9]_. The
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following is a list of features provided by argparse but not present
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in getopt or optparse:
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* While it is true there are two *option* parsing libraries, there
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are no full command line parsing libraries -- both getopt and
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optparse support only options and have no support for positional
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arguments. The argparse module handles both, and as a result, is
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able to generate better help messages, avoiding redundancies like
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the ``usage=`` string usually required by optparse.
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* The argparse module values practicality over purity. Thus, argparse
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allows required options and customization of which characters are
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used to identify options, while optparse explicitly states "the
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phrase 'required option' is self-contradictory" and that the option
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syntaxes ``-pf``, ``-file``, ``+f``, ``+rgb``, ``/f`` and ``/file``
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"are not supported by optparse, and they never will be".
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* The argparse module allows options to accept a variable number of
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arguments using ``nargs='?'``, ``nargs='*'`` or ``nargs='+'``. The
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optparse module provides an untested recipe for some part of this
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functionality [10]_ but admits that "things get hairy when you want
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an option to take a variable number of arguments."
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* The argparse module supports subcommands, where a main command
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line parser dispatches to other command line parsers depending on
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the command line arguments. This is a common pattern in command
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line interfaces, e.g. ``svn co`` and ``svn up``.
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Why isn't the functionality just being added to optparse?
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=========================================================
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Clearly all the above features offer improvements over what is
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available through optparse. A reasonable question then is why these
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features are not simply provided as patches to optparse, instead of
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introducing an entirely new module. In fact, the original development
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of argparse intended to do just that, but because of various fairly
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constraining design decisions of optparse, this wasn't really
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possible. Some of the problems included:
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* The optparse module exposes the internals of its parsing algorithm.
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In particular, ``parser.largs`` and ``parser.rargs`` are guaranteed
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to be available to callbacks [11]_. This makes it extremely
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difficult to improve the parsing algorithm as was necessary in
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argparse for proper handling of positional arguments and variable
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length arguments. For example, ``nargs='+'`` in argparse is matched
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using regular expressions and thus has no notion of things like
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``parser.largs``.
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* The optparse extension APIs are extremely complex. For example,
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just to use a simple custom string-to-object conversion function,
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you have to subclass ``Option``, hack class attributes, and then
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specify your custom option type to the parser, like this::
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class MyOption(Option):
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TYPES = Option.TYPES + ("mytype",)
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TYPE_CHECKER = copy(Option.TYPE_CHECKER)
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TYPE_CHECKER["mytype"] = check_mytype
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parser = optparse.OptionParser(option_class=MyOption)
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parser.add_option("-m", type="mytype")
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For comparison, argparse simply allows conversion functions to be
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used as ``type=`` arguments directly, e.g.::
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parser = argparse.ArgumentParser()
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parser.add_option("-m", type=check_mytype)
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But given the baroque customization APIs of optparse, it is unclear
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how such a feature should interact with those APIs, and it is
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quite possible that introducing the simple argparse API would break
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existing custom Option code.
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* Both optparse and argparse parse command line arguments and assign
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them as attributes to an object returned by ``parse_args``.
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However, the optparse module guarantees that the ``take_action``
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method of custom actions will always be passed a ``values`` object
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which provides an ``ensure_value`` method [12]_, while the argparse
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module allows attributes to be assigned to any object, e.g.::
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foo_object = ...
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parser.parse_args(namespace=foo_object)
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foo_object.some_attribute_parsed_from_command_line
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Modifying optparse to allow any object to be passed in would be
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difficult because simply passing the ``foo_object`` around instead
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of a ``Values`` instance will break existing custom actions that
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depend on the ``ensure_value`` method.
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Because of issues like these, which made it unreasonably difficult
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for argparse to stay compatible with the optparse APIs, argparse was
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developed as an independent module. Given these issues, merging all
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the argparse features into optparse with no backwards
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incompatibilities seems unlikely.
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Deprecation of getopt and optparse
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==================================
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There is still some debate over the best way (if at all) to encourage
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users to move from getopt and optparse to their replacement,
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argparse. The current recommendation of this PEP is the following
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conservative deprecation strategy:
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* Python 3.2, Python 2.7 and any later Python 2.X releases --
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PendingDeprecation warnings, which by default are not displayed,
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and documentation notes directing users of getopt and optparse to
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argparse.
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* Python 3.3 -- Same as above
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* Python 3.4 -- Deprecation warnings for getopt and optparse, which
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by default *are* displayed.
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Though this is slower than the usual deprecation process, it seems
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prudent to avoid producing any casually visible Deprecation warnings
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until Python 3.X has had some additional time to attract developers.
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Open Issues
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===========
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The argparse module supports Python from 2.3 up through 3.2 and as a
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result relies on traditional ``%(foo)s`` style string formatting. It
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has been suggested that it might be better to use the new style
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``{foo}`` string formatting [13]_. This seems like a good idea, but
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would break backwards compatibility for existing argparse-based
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scripts unless we can come up with a way to reasonably support both
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syntaxes.
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References
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==========
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.. [1] argparse
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(http://code.google.com/p/argparse/)
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.. [2] getopt
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(http://docs.python.org/library/getopt.html)
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.. [3] optparse
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(http://docs.python.org/library/optparse.html)
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.. [4] argparse in IPython
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(http://mail.scipy.org/pipermail/ipython-dev/2009-April/005102.html)
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.. [5] argparse in Debian
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(http://packages.debian.org/search?keywords=python-argparse)
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.. [6] 2007-01-03 request for argparse in the standard library
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(http://mail.python.org/pipermail/python-list/2007-January/592646.html)
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.. [7] 2009-06-09 request for argparse in the standard library
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(http://bugs.python.org/issue6247)
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.. [8] 2009-09-10 request for argparse in the standard library
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(http://mail.python.org/pipermail/stdlib-sig/2009-September/000342.html)
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.. [9] Fredrik Lundh response to [6]_
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(http://mail.python.org/pipermail/python-list/2007-January/592675.html)
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.. [10] optparse variable args
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(http://docs.python.org/library/optparse.html#callback-example-6-variable-arguments)
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.. [11] parser.largs and parser.rargs
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(http://docs.python.org/library/optparse.html#how-callbacks-are-called)
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.. [12] take_action values argument
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(http://docs.python.org/library/optparse.html#adding-new-actions)
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.. [13] use {}-formatting instead of %-formatting
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(http://bugs.python.org/msg89279)
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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