Sorry for the delay, but I've finally updated PEP 389, the argparse PEP, based on all the feedback from python-dev. The full PEP is below, but in short, the important changes are: * The getopt module will *not* be deprecated. * In Python 2, the -3 flag will cause deprecation warnings to be issued for optparse. * No casually visible deprecation warnings for optparse are expected until Jun 2013. * Support for {}-format strings will be added when one of the converters is approved. This can be done at any point though, so it shouldn't hold up inclusion of argparse. Steve ---------------------------------------------------------------------- PEP: 389 Title: argparse - new command line parsing module Version: $Revision: 75674 $ Last-Modified: $Date: 2009-10-24 12:01:49 -0700 (Sat, 24 Oct 2009) $ Author: Steven Bethard <steven.bethard@gmail.com> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 25-Sep-2009 Python-Version: 2.7 and 3.2 Post-History: 27-Sep-2009, 24-Oct-2009 Abstract ======== This PEP proposes inclusion of the argparse [1]_ module in the Python standard library in Python 2.7 and 3.2. Motivation ========== The argparse module is a command line parsing library which provides more functionality than the existing command line parsing modules in the standard library, getopt [2]_ and optparse [3]_. It includes support for positional arguments (not just options), subcommands, required options, options syntaxes like "/f" and "+rgb", zero-or-more and one-or-more style arguments, and many other features the other two lack. The argparse module is also already a popular third-party replacement for these modules. It is used in projects like IPython (the Scipy Python shell) [4]_, is included in Debian testing and unstable [5]_, and since 2007 has had various requests for its inclusion in the standard library [6]_ [7]_ [8]_. This popularity suggests it may be a valuable addition to the Python libraries. Why aren't getopt and optparse enough? ====================================== One argument against adding argparse is that thare are "already two different option parsing modules in the standard library" [9]_. The following is a list of features provided by argparse but not present in getopt or optparse: * While it is true there are two *option* parsing libraries, there are no full command line parsing libraries -- both getopt and optparse support only options and have no support for positional arguments. The argparse module handles both, and as a result, is able to generate better help messages, avoiding redundancies like the ``usage=`` string usually required by optparse. * The argparse module values practicality over purity. Thus, argparse allows required options and customization of which characters are used to identify options, while optparse explicitly states "the phrase 'required option' is self-contradictory" and that the option syntaxes ``-pf``, ``-file``, ``+f``, ``+rgb``, ``/f`` and ``/file`` "are not supported by optparse, and they never will be". * The argparse module allows options to accept a variable number of arguments using ``nargs='?'``, ``nargs='*'`` or ``nargs='+'``. The optparse module provides an untested recipe for some part of this functionality [10]_ but admits that "things get hairy when you want an option to take a variable number of arguments." * The argparse module supports subcommands, where a main command line parser dispatches to other command line parsers depending on the command line arguments. This is a common pattern in command line interfaces, e.g. ``svn co`` and ``svn up``. Why isn't the functionality just being added to optparse? ========================================================= Clearly all the above features offer improvements over what is available through optparse. A reasonable question then is why these features are not simply provided as patches to optparse, instead of introducing an entirely new module. In fact, the original development of argparse intended to do just that, but because of various fairly constraining design decisions of optparse, this wasn't really possible. Some of the problems included: * The optparse module exposes the internals of its parsing algorithm. In particular, ``parser.largs`` and ``parser.rargs`` are guaranteed to be available to callbacks [11]_. This makes it extremely difficult to improve the parsing algorithm as was necessary in argparse for proper handling of positional arguments and variable length arguments. For example, ``nargs='+'`` in argparse is matched using regular expressions and thus has no notion of things like ``parser.largs``. * The optparse extension APIs are extremely complex. For example, just to use a simple custom string-to-object conversion function, you have to subclass ``Option``, hack class attributes, and then specify your custom option type to the parser, like this:: class MyOption(Option): TYPES = Option.TYPES + ("mytype",) TYPE_CHECKER = copy(Option.TYPE_CHECKER) TYPE_CHECKER["mytype"] = check_mytype parser = optparse.OptionParser(option_class=MyOption) parser.add_option("-m", type="mytype") For comparison, argparse simply allows conversion functions to be used as ``type=`` arguments directly, e.g.:: parser = argparse.ArgumentParser() parser.add_option("-m", type=check_mytype) But given the baroque customization APIs of optparse, it is unclear how such a feature should interact with those APIs, and it is quite possible that introducing the simple argparse API would break existing custom Option code. * Both optparse and argparse parse command line arguments and assign them as attributes to an object returned by ``parse_args``. However, the optparse module guarantees that the ``take_action`` method of custom actions will always be passed a ``values`` object which provides an ``ensure_value`` method [12]_, while the argparse module allows attributes to be assigned to any object, e.g.:: foo_object = ... parser.parse_args(namespace=foo_object) foo_object.some_attribute_parsed_from_command_line Modifying optparse to allow any object to be passed in would be difficult because simply passing the ``foo_object`` around instead of a ``Values`` instance will break existing custom actions that depend on the ``ensure_value`` method. Because of issues like these, which made it unreasonably difficult for argparse to stay compatible with the optparse APIs, argparse was developed as an independent module. Given these issues, merging all the argparse features into optparse with no backwards incompatibilities seems unlikely. Deprecation of optparse ======================= Because all of optparse's features are available in argparse, the optparse module will be deprecated. The following very conservative deprecation strategy will be followed: * Python 2.7+ and 3.2+ -- The following note will be added to the optparse documentation: The optparse module is deprecated, and has been replaced by the argparse module. * Python 2.7+ -- If the Python 3 compatibility flag, ``-3``, is provided at the command line, then importing optparse will issue a DeprecationWarning. Otherwise no warnings will be issued. * Python 3.2 (estimated Jun 2010) -- Importing optparse will issue a PendingDeprecationWarning, which is not displayed by default. * Python 3.3 (estimated Jan 2012) -- Importing optparse will issue a PendingDeprecationWarning, which is not displayed by default. * Python 3.4 (estimated Jun 2013) -- Importing optparse will issue a DeprecationWarning, which *is* displayed by default. * Python 3.5 (estimated Jan 2015) -- The optparse module will be removed. Though this is slower than the usual deprecation process, with two releases of PendingDeprecationWarnings instead of the usual one, it seems prudent to avoid producing any casually visible Deprecation warnings until Python 3.X has had some additional time to attract developers. Updates to getopt documentation =============================== The getopt module will not be deprecated. However, its documentation will be updated to point to argparse in a couple of places. At the top of the module, the following note will be added: The getopt module is a parser for command line options whose API is designed to be familiar to users of the C getopt function. Users who are unfamiliar with the C getopt function or who would like to write less code and get better help and error messages should consider using the argparse module instead. Additionally, after the final getopt example, the following note will be added: Note that an equivalent command line interface could be produced with less code by using the argparse module:: import argparse if __name__ == '__main__': parser = argparse.ArgumentParser() parser.add_argument('-o', '--output') parser.add_argument('-v', dest='verbose', action='store_true') args = parser.parse_args() # ... do something with args.output ... # ... do something with args.verbose .. Deferred: string formatting =========================== The argparse module supports Python from 2.3 up through 3.2 and as a result relies on traditional ``%(foo)s`` style string formatting. It has been suggested that it might be better to use the new style ``{foo}`` string formatting [13]_. There was some discussion about how best to do this for modules in the standard library [14]_ and several people are developing functions for automatically converting %-formatting to {}-formatting [15]_ [16]_. When one of these is added to the standard library, argparse will use them to support both formatting styles. Rejected: getopt compatibility methods ====================================== Previously, when this PEP was suggesting the deprecation of getopt as well as optparse, there was some talk of adding a method like:: ArgumentParser.add_getopt_arguments(options[, long_options]) However, this method will not be added for a number of reasons: * The getopt module is not being deprecated, so there is less need. * This method would not actually ease the transition for any getopt users who were already maintaining usage messages, because the API above gives no way of adding help messages to the arguments. * Some users of getopt consider it very important that only a single function call is necessary. The API above does not satisfy this requirement because both ``ArgumentParser()`` and ``parse_args()`` must also be called. Out of Scope: Various Feature Requests ====================================== Several feature requests for argparse were made in the discussion of this PEP: * Support argument defaults from environment variables * Support argument defaults from configuration files * Support "foo --help subcommand" in addition to the currently supported "foo subcommand --help" These are all reasonable feature requests for the argparse module, but are out of the scope of this PEP, and have been redirected to the argparse issue tracker. Discussion: sys.err and sys.exit ================================ There were some concerns that argparse by default always writes to ``sys.err`` and always calls ``sys.exit`` when invalid arguments are provided. This is the desired behavior for the vast majority of argparse use cases which revolve around simple command line interfaces. However, in some cases, it may be desirable to keep argparse from exiting, or to have it write its messages to something other than ``sys.err``. These use cases can be supported by subclassing ``ArgumentParser`` and overriding the ``exit`` or ``_print_message`` methods. The latter is an undocumented implementation detail, but could be officially exposed if this turns out to be a common need. References ========== .. [1] argparse (http://code.google.com/p/argparse/) .. [2] getopt (http://docs.python.org/library/getopt.html) .. [3] optparse (http://docs.python.org/library/optparse.html) .. [4] argparse in IPython (http://mail.scipy.org/pipermail/ipython-dev/2009-April/005102.html) .. [5] argparse in Debian (http://packages.debian.org/search?keywords=python-argparse) .. [6] 2007-01-03 request for argparse in the standard library (http://mail.python.org/pipermail/python-list/2007-January/592646.html) .. [7] 2009-06-09 request for argparse in the standard library (http://bugs.python.org/issue6247) .. [8] 2009-09-10 request for argparse in the standard library (http://mail.python.org/pipermail/stdlib-sig/2009-September/000342.html) .. [9] Fredrik Lundh response to [6]_ (http://mail.python.org/pipermail/python-list/2007-January/592675.html) .. [10] optparse variable args (http://docs.python.org/library/optparse.html#callback-example-6-variable-arg...) .. [11] parser.largs and parser.rargs (http://docs.python.org/library/optparse.html#how-callbacks-are-called) .. [12] take_action values argument (http://docs.python.org/library/optparse.html#adding-new-actions) .. [13] use {}-formatting instead of %-formatting (http://bugs.python.org/msg89279) .. [14] transitioning from % to {} formatting (http://mail.python.org/pipermail/python-dev/2009-September/092326.html) .. [15] Vinay Sajip's %-to-{} converter (http://gist.github.com/200936) .. [16] Benjamin Peterson's %-to-{} converter (http://bazaar.launchpad.net/~gutworth/+junk/mod2format/files) Copyright ========= This document has been placed in the public domain.  .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: