fix: tolerate None __file__ in _distutils_hack frame detector

_distutils_hack/__init__.py

@@ -175,8 +175,12 @@ def frame_file_is_setup(frame):
         """
         Return True if the indicated frame suggests a setup.py file.
         """
-        # some frames may not have __file__ (#2940)
-        return frame.f_globals.get('__file__', '').endswith('setup.py')
+        # some frames may not have __file__ (#2940);
+        # on Python 3.14+ __file__ may also be present but None, which would
+        # crash on the default-value fallback. Treat None and non-strings the
+        # same as missing. #5263
+        filename = frame.f_globals.get('__file__')
+        return isinstance(filename, str) and filename.endswith('setup.py')
 
     def spec_for_sensitive_tests(self):
         """

newsfragments/5263.bugfix.rst

@@ -0,0 +1 @@
+Guard against __file__ being None in _distutils_hack's frame detector so pip startup no longer crashes on Python 3.14 when a frame's globals carry a None __file__.

setuptools/tests/test_distutils_adoption.py

@@ -86,6 +86,32 @@ def test_pip_import(venv):
     venv.run(cmd, **_TEXT_KWARGS)
 
 
+def test_frame_file_is_setup_handles_missing_file():
+    """
+    _distutils_hack.DistutilsMetaFinder.frame_file_is_setup should tolerate
+    frames whose f_globals does not have __file__ (#2940) or has __file__ set
+    to None, which can happen on Python 3.14+ during pip startup. #5263
+    """
+    from _distutils_hack import DistutilsMetaFinder
+
+    class Frame:
+        def __init__(self, filename):
+            self._globals = {"__file__": filename}
+
+        @property
+        def f_globals(self):
+            return self._globals
+
+    # Missing __file__: treated as not a setup.py
+    assert not DistutilsMetaFinder.frame_file_is_setup(Frame("MISSING"))
+    # __file__ is None (Python 3.14+ can land here): no crash, treated as not setup.py
+    assert not DistutilsMetaFinder.frame_file_is_setup(Frame(None))
+    # __file__ ends with setup.py: still detected
+    assert DistutilsMetaFinder.frame_file_is_setup(Frame("/some/dir/setup.py"))
+    # __file__ is something else: not a setup.py
+    assert not DistutilsMetaFinder.frame_file_is_setup(Frame("/some/dir/main.py"))
+
+
 def test_distutils_has_origin():
     """
     Distutils module spec should have an origin. #2990.