gh-158985: Make profile and cProfile -m run the module as __main__

Pass alter_sys=True to runpy.run_module() in the -m branch of profile
and profiling.tracing, so that sys.modules['__main__'] and sys.argv[0]
refer to the profiled module, as with python -m.

Author: sricursion

Lib/profile.py

@@ -590,7 +590,7 @@ def main():
     if len(args) > 0:
         if options.module:
             import runpy
-            code = "run_module(modname, run_name='__main__')"
+            code = "run_module(modname, run_name='__main__', alter_sys=True)"
             globs = {
                 'run_module': runpy.run_module,
                 'modname': args[0]

Lib/profiling/tracing/__init__.py

@@ -176,7 +176,7 @@ def main():
 
     if len(args) > 0:
         if options.module:
-            code = "run_module(modname, run_name='__main__')"
+            code = "run_module(modname, run_name='__main__', alter_sys=True)"
             globs = {
                 'run_module': runpy.run_module,
                 'modname': args[0]

Lib/test/test_profile.py

@@ -4,6 +4,7 @@
 import pstats
 import unittest
 import os
+import textwrap
 import warnings
 from difflib import unified_diff
 from io import StringIO
@@ -15,7 +16,8 @@
     warnings.simplefilter("ignore", DeprecationWarning)
     import profile
 from test.profilee import testfunc, timer
-from test.support.script_helper import assert_python_failure, assert_python_ok
+from test.support.script_helper import (
+    assert_python_failure, assert_python_ok, make_script)
 
 
 class ProfileTest(unittest.TestCase):
@@ -120,6 +122,23 @@ def test_run_profile_as_module(self):
         assert_python_ok('-m', self.profilermodule.__name__,
                          '-m', 'timeit', '-n', '1')
 
+    def test_profile_module_as_main(self):
+        # The module must run as __main__, like with "python -m module".
+        with temp_dir() as tmpdir:
+            make_script(tmpdir, 'profiled_module', textwrap.dedent("""\
+                import pickle
+                import sys
+
+                class Foo:
+                    pass
+
+                assert sys.modules['__main__'].Foo is Foo
+                assert sys.argv[0].endswith('profiled_module.py')
+                pickle.dumps(Foo())
+                """))
+            assert_python_ok('-m', self.profilermodule.__name__,
+                             '-m', 'profiled_module', PYTHONPATH=tmpdir)
+
     def test_output_file_when_changing_directory(self):
         with temp_dir() as tmpdir, change_cwd(tmpdir):
             os.mkdir('dest')

Misc/NEWS.d/next/Library/2026-10-08-01-32-15.gh-issue-158985.tKnysx.rst

@@ -0,0 +1,5 @@
+Fix :mod:`cProfile` and :mod:`profile` when profiling a module with the
+``-m`` option: the module now runs as ``__main__`` and ``sys.argv[0]`` is
+set to its path, as with ``python -m``. This fixes errors when pickling
+objects defined in the module, for example when using :mod:`multiprocessing`
+with the ``spawn`` start method.