[3.15] GH-155009: Preserve argparse prog for programmatic module-as-main execution (GH-155199) (#155200)

GH-155009: Preserve argparse prog for programmatic module-as-main execution (GH-155199)
(cherry picked from commit ca4518b)

Co-authored-by: Savannah Ostrowski <savannah@python.org>

Author: miss-islington

Lib/argparse.py

@@ -1968,6 +1968,9 @@ def _prog_name(prog=None):
     if modspec is None:
         # simple script
         return _os.path.basename(arg0)
+    if modspec.name != '__main__' and arg0 != modspec.origin:
+        # named module executed as main without altering sys.argv[0]
+        return _os.path.basename(arg0)
     py = _os.path.basename(_sys.executable)
     if modspec.name != '__main__':
         # imported module or package

Lib/test/test_argparse.py

@@ -7341,6 +7341,19 @@ def test_module(self, compiled=False):
     def test_module_compiled(self):
         self.test_module(compiled=True)
 
+    def test_module_as_main_without_altering_argv(self):
+        basename = 'module' + os_helper.FS_NONASCII
+        modulename = f'{self.dirname}.{basename}'
+        self.make_script(self.dirname, basename)
+        runner_source = textwrap.dedent(f'''\
+            import runpy
+            runpy._run_module_as_main({modulename!r}, alter_argv=False)
+        ''')
+        runner = script_helper.make_script(
+            self.dirname, 'runner', runner_source)
+        self.check_usage(os.path.basename(runner), runner,
+                         PYTHONPATH=os.curdir)
+
     def test_package(self, compiled=False):
         basename = 'subpackage' + os_helper.FS_NONASCII
         packagename = f'{self.dirname}.{basename}'

Misc/NEWS.d/next/Library/2026-08-05-02-36-18.gh-issue-155009.YiJAiP.rst

@@ -0,0 +1,3 @@
+Fix :class:`argparse.ArgumentParser` to preserve the program name from
+``sys.argv[0]`` when a named module is executed as the main program without
+replacing ``sys.argv[0]``.