Only test against filenames with ".py" stripped.

Author: serhiy-storchaka

Doc/library/warnings.rst

@@ -488,14 +488,13 @@ Available Functions
 
    *module*, if supplied, should be the module name.
    If no module is passed, the module regular expression in
-   :ref:`warnings filter <warning-filter>` will be tested against the filename
-   with ``/__init__.py`` and ``.py`` (and ``.pyw`` on Windows) stripped and
-   against the module names constructed from the path components starting
-   from all parent directories.
-   For example, when filename is ``'/path/to/package/module.py'``, it will
-   be tested against ``'/path/to/package/module'``,
-   ``'path.to.package.module'``, ``'to.package.module'``
-   ``'package.module'`` and ``'module'``.
+   :ref:`warnings filter <warning-filter>` will be tested against the module
+   names constructed from the path components starting from all parent
+   directories (with ``/__init__.py``, ``.py`` and, on Windows, ``.pyw``
+   stripped) and against the filename with ``.py`` stripped.
+   For example, when the filename is ``'/path/to/package/module.py'``, it will
+   be tested against  ``'path.to.package.module'``, ``'to.package.module'``
+   ``'package.module'``, ``'module'``, and ``'/path/to/package/module'``.
 
    *registry*, if supplied, should be the ``__warningregistry__`` dictionary
    of the module.

Doc/whatsnew/3.15.rst

@@ -608,8 +608,8 @@ warnings
   argument is passed.
   It now tests the module regular expression in the warnings filter not only
   against the filename with ``.py`` stripped, but also against module names
-  constructed starting from different parent directories of the filename.
-  Strip also ``.pyw`` (on Windows) and ``/__init__.py``.
+  constructed starting from different parent directories of the filename
+  (with ``/__init__.py``, ``.py`` and, on Windows, ``.pyw`` stripped).
   (Contributed by Serhiy Storchaka in :gh:`135801`.)
 
 

Lib/_py_warnings.py

@@ -526,27 +526,22 @@ def _match_filename(pattern, filename, *, MS_WINDOWS=(sys.platform == 'win32')):
     if filename[0] == '<' and filename[-1] == '>':
         return pattern.match(filename) is not None
 
+    is_py = (filename[-3:].lower() == '.py'
+             if MS_WINDOWS else
+             filename.endswith('.py'))
+    if is_py:
+        filename = filename[:-3]
+    if pattern.match(filename):  # for backward compatibility
+        return True
     if MS_WINDOWS:
-        if filename[-12:].lower() in (r'\__init__.py', '/__init__.py'):
-            if pattern.match(filename[:-3]): # without '.py'
-                return True
-            filename = filename[:-12]
-        elif filename[-3:].lower() == '.py':
-            filename = filename[:-3]
-        elif filename[-4:].lower() == '.pyw':
+        if is_py and filename[-9:].lower() in (r'\__init__', '/__init__'):
+            filename = filename[:-9]
+        elif not is_py and filename[-4:].lower() == '.pyw':
             filename = filename[:-4]
-        if pattern.match(filename):
-            return True
-        filename = filename.replace('\\', '/')
+        filename = filename.replace('\\', '.')
     else:
-        if filename.endswith('/__init__.py'):
-            if pattern.match(filename[:-3]): # without '.py'
-                return True
-            filename = filename[:-12]
-        elif filename.endswith('.py'):
-            filename = filename[:-3]
-        if pattern.match(filename):
-            return True
+        if is_py and filename.endswith('/__init__'):
+            filename = filename[:-9]
     filename = filename.replace('/', '.')
     i = 0
     while True:

Lib/test/test_warnings/__init__.py

@@ -289,10 +289,10 @@ def test_filter_module(self):
             if MS_WINDOWS:
                 self.module.warn_explicit('msg', UserWarning, r'/path/to/package/module.PY', 42)
                 self.assertEqual(len(w), 3)
-                self.module.warn_explicit('msg', UserWarning, r'/path/to/package/module/__INIT__.PY', 42)
-                self.assertEqual(len(w), 4)
-                self.module.warn_explicit('msg', UserWarning, r'/path/to/package/module.PYW', 42)
-                self.assertEqual(len(w), 5)
+                with self.assertRaises(UserWarning):
+                    self.module.warn_explicit('msg', UserWarning, r'/path/to/package/module/__init__.py', 42)
+                with self.assertRaises(UserWarning):
+                    self.module.warn_explicit('msg', UserWarning, r'/path/to/package/module.pyw', 42)
                 with self.assertRaises(UserWarning):
                     self.module.warn_explicit('msg', UserWarning, r'\path\to\package\module', 42)
 
@@ -314,16 +314,14 @@ def test_filter_module(self):
                 self.assertEqual(len(w), 2)
                 self.module.warn_explicit('msg', UserWarning, r'C:\path\to\package\module.PY', 42)
                 self.assertEqual(len(w), 3)
-                self.module.warn_explicit('msg', UserWarning, r'C:\path\to\package\module\__INIT__.PY', 42)
-                self.assertEqual(len(w), 4)
-                self.module.warn_explicit('msg', UserWarning, r'C:\path\to\package\module.PYW', 42)
-                self.assertEqual(len(w), 5)
+                with self.assertRaises(UserWarning):
+                    self.module.warn_explicit('msg', UserWarning, r'C:\path\to\package\module.pyw', 42)
                 with self.assertRaises(UserWarning):
                     self.module.warn_explicit('msg', UserWarning, r'C:\PATH\TO\PACKAGE\MODULE', 42)
                 with self.assertRaises(UserWarning):
                     self.module.warn_explicit('msg', UserWarning, r'C:/path/to/package/module', 42)
                 with self.assertRaises(UserWarning):
-                    self.module.warn_explicit('msg', UserWarning, r'C:\path\to\package\module\__init__', 42)
+                    self.module.warn_explicit('msg', UserWarning, r'C:\path\to\package\module\__init__.py', 42)
 
         with self.module.catch_warnings(record=True) as w:
             self.module.simplefilter('error')

Misc/NEWS.d/next/Library/2025-10-16-17-17-20.gh-issue-135801.faH3fa.rst

@@ -2,5 +2,6 @@ Improve filtering by module in :func:`warnings.warn_explicit` if no *module*
 argument is passed. It now tests the module regular expression in the
 warnings filter not only against the filename with ``.py`` stripped, but
 also against module names constructed starting from different parent
-directories of the filename. Strip also ``.pyw`` (on Windows) and
+directories of the filename (with ``/__init__.py``, ``.py`` and, on Windows,
+``.pyw`` stripped).
 ``/__init__.py``.