[3.12] GH-124478: Cleanup argparse documentation (GH-124877)

Doc/howto/argparse-optparse.rst

@@ -0,0 +1,55 @@
+.. currentmodule:: argparse
+
+.. _upgrading-optparse-code:
+
+==========================
+Upgrading optparse code
+==========================
+
+Originally, the :mod:`argparse` module had attempted to maintain compatibility
+with :mod:`optparse`.  However, :mod:`optparse` was difficult to extend
+transparently, particularly with the changes required to support
+``nargs=`` specifiers and better usage messages.  When most everything in
+:mod:`optparse` had either been copy-pasted over or monkey-patched, it no
+longer seemed practical to try to maintain the backwards compatibility.
+
+The :mod:`argparse` module improves on the :mod:`optparse`
+module in a number of ways including:
+
+* Handling positional arguments.
+* Supporting subcommands.
+* Allowing alternative option prefixes like ``+`` and ``/``.
+* Handling zero-or-more and one-or-more style arguments.
+* Producing more informative usage messages.
+* Providing a much simpler interface for custom ``type`` and ``action``.
+
+A partial upgrade path from :mod:`optparse` to :mod:`argparse`:
+
+* Replace all :meth:`optparse.OptionParser.add_option` calls with
+  :meth:`ArgumentParser.add_argument` calls.
+
+* Replace ``(options, args) = parser.parse_args()`` with ``args =
+  parser.parse_args()`` and add additional :meth:`ArgumentParser.add_argument`
+  calls for the positional arguments. Keep in mind that what was previously
+  called ``options``, now in the :mod:`argparse` context is called ``args``.
+
+* Replace :meth:`optparse.OptionParser.disable_interspersed_args`
+  by using :meth:`~ArgumentParser.parse_intermixed_args` instead of
+  :meth:`~ArgumentParser.parse_args`.
+
+* Replace callback actions and the ``callback_*`` keyword arguments with
+  ``type`` or ``action`` arguments.
+
+* Replace string names for ``type`` keyword arguments with the corresponding
+  type objects (e.g. int, float, complex, etc).
+
+* Replace :class:`optparse.Values` with :class:`Namespace` and
+  :exc:`optparse.OptionError` and :exc:`optparse.OptionValueError` with
+  :exc:`ArgumentError`.
+
+* Replace strings with implicit arguments such as ``%default`` or ``%prog`` with
+  the standard Python syntax to use dictionaries to format strings, that is,
+  ``%(default)s`` and ``%(prog)s``.
+
+* Replace the OptionParser constructor ``version`` argument with a call to
+  ``parser.add_argument('--version', action='version', version='<the version>')``.