gh-66436: Improved prog default value for argparse.ArgumentParser (GH-124799)

It can now have one of three forms:

* basename(argv0) -- for simple scripts
* python arv0 -- for directories, ZIP files, etc
* python -m module -- for imported modules

Co-authored-by: Alyssa Coghlan <ncoghlan@gmail.com>

2 files changed, 39 insertions, 9 deletions

diff --git a/Doc/library/argparse.rst b/Doc/library/argparse.rst
index a4683bc..83d0a9e 100644
--- a/Doc/library/argparse.rst
+++ b/Doc/library/argparse.rst
@@ -30,7 +30,7 @@ Quick Links for ArgumentParser
 ========================= =========================================================================================================== ==================================================================================
 Name                      Description                                                                                                 Values
 ========================= =========================================================================================================== ==================================================================================
-prog_                     The name of the program                                                                                     Defaults to ``os.path.basename(sys.argv[0])``
+prog_                     The name of the program
 usage_                    The string describing the program usage
 description_              A brief description of what the program does
 epilog_                   Additional description of the program after the argument help
@@ -214,8 +214,8 @@ ArgumentParser objects
    as keyword arguments. Each parameter has its own more detailed description
    below, but in short they are:
 
-   * prog_ - The name of the program (default:
-     ``os.path.basename(sys.argv[0])``)
+   * prog_ - The name of the program (default: generated from the ``__main__``
+     module attributes and ``sys.argv[0]``)
 
    * usage_ - The string describing the program usage (default: generated from
      arguments added to parser)
@@ -268,10 +268,18 @@ The following sections describe how each of these are used.
 prog
 ^^^^
 
-By default, :class:`ArgumentParser` objects use the base name
-(see :func:`os.path.basename`) of ``sys.argv[0]`` to determine
-how to display the name of the program in help messages.  This default is almost
-always desirable because it will make the help messages match the name that was
+By default, :class:`ArgumentParser` calculates the name of the program
+to display in help messages depending on the way the Python inerpreter was run:
+
+* The :func:`base name <os.path.basename>` of ``sys.argv[0]`` if a file was
+  passed as argument.
+* The Python interpreter name followed by ``sys.argv[0]`` if a directory or
+  a zipfile was passed as argument.
+* The Python interpreter name followed by ``-m`` followed by the
+  module or package name if the :option:`-m` option was used.
+
+This default is almost
+always desirable because it will make the help messages match the string that was
 used to invoke the program on the command line.  For example, consider a file
 named ``myprogram.py`` with the following code::
 
@@ -281,7 +289,7 @@ named ``myprogram.py`` with the following code::
    args = parser.parse_args()
 
 The help for this program will display ``myprogram.py`` as the program name
-(regardless of where the program was invoked from):
+(regardless of where the program was invoked from) if it is run as a script:
 
 .. code-block:: shell-session
 
@@ -299,6 +307,17 @@ The help for this program will display ``myprogram.py`` as the program name
     -h, --help  show this help message and exit
     --foo FOO   foo help
 
+If it is executed via the :option:`-m` option, the help will display a corresponding command line:
+
+.. code-block:: shell-session
+
+   $ /usr/bin/python3 -m subdir.myprogram --help
+   usage: python3 -m subdir.myprogram [-h] [--foo FOO]
+
+   options:
+    -h, --help  show this help message and exit
+    --foo FOO   foo help
+
 To change this default behavior, another value can be supplied using the
 ``prog=`` argument to :class:`ArgumentParser`::
 
@@ -309,7 +328,8 @@ To change this default behavior, another value can be supplied using the
    options:
     -h, --help  show this help message and exit
 
-Note that the program name, whether determined from ``sys.argv[0]`` or from the
+Note that the program name, whether determined from ``sys.argv[0]``,
+from the ``__main__`` module attributes or from the
 ``prog=`` argument, is available to help messages using the ``%(prog)s`` format
 specifier.
 
@@ -324,6 +344,9 @@ specifier.
     -h, --help  show this help message and exit
     --foo FOO   foo of the myprogram program
 
+.. versionchanged:: 3.14
+   The default ``prog`` value now reflects how ``__main__`` was actually executed,
+   rather than always being ``os.path.basename(sys.argv[0])``.
 
 usage
 ^^^^^
diff --git a/Doc/whatsnew/3.14.rst b/Doc/whatsnew/3.14.rst
index ffc0012..67d8d38 100644
--- a/Doc/whatsnew/3.14.rst
+++ b/Doc/whatsnew/3.14.rst
@@ -202,6 +202,13 @@ New Modules
 Improved Modules
 ================
 
+argparse
+--------
+
+* The default value of the :ref:`program name <prog>` for
+  :class:`argparse.ArgumentParser` now reflects the way the Python
+  interpreter was instructed to find the ``__main__`` module code.
+  (Contributed by Serhiy Storchaka and Alyssa Coghlan in :gh:`66436`.)
 
 ast
 ---