Only return PATHEXT matches on win32 if X_OK is in mode

Lib/shutil.py

@@ -1551,17 +1551,19 @@ def which(cmd, mode=os.F_OK | os.X_OK, path=None):
         pathext_source = os.getenv("PATHEXT") or _WIN_DEFAULT_PATHEXT
         pathext = [ext for ext in pathext_source.split(os.pathsep) if ext]
 
-        dot = '.'
         if use_bytes:
             pathext = [os.fsencode(ext) for ext in pathext]
-            dot = b'.'
-
-        # Attempt to match CMD behavior:
-        # Only try the given cmd if it has an extension (therefore has a dot)
-        # or a dot is a pathext in PATHEXT.
-        # Otherwise use PATHEXT to formulate paths to check.
-        files = (([cmd] if (dot in cmd or dot in pathext) else []) +
-                 [cmd + ext for ext in pathext])
+
+        files = ([cmd] + [cmd + ext for ext in pathext])
+
+        # gh-109590. If we are looking for an executable, we need to look
+        # for a PATHEXT match. The first cmd is the direct match
+        # (e.g. python.exe instead of python)
+        # Check that direct match first if and only if the extension is in PATHEXT
+        if mode & os.X_OK and not any(
+            [os.path.splitext(files[0])[1].upper() == ext.upper() for ext in pathext]
+            ):
+            files = files[1:]
     else:
         # On other platforms you don't have things like PATHEXT to tell you
         # what file suffixes are executable, so just pass on cmd as-is.

Lib/test/test_shutil.py

@@ -2323,34 +2323,40 @@ def test_win_path_needs_curdir(self):
 
     # See GH-109590
     @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
-    def test_extensionless_file_resolution_no_dot_in_pathext(self):
+    def test_pathext_enforced_for_execute(self):
         with os_helper.EnvironmentVarGuard() as env:
-            env['PATHEXT'] = ".test;"
-            env['PATH'] = self.temp_dir if isinstance(self.temp_dir, str) else self.temp_dir.decode()
-
-            extensionless_file_in_path = os.path.join(self.temp_dir, self.to_text_type("file"))
-            open(extensionless_file_in_path, 'w').close()
+            env["PATH"] = self.temp_dir if isinstance(self.temp_dir, str) else self.temp_dir.decode()
+            env["PATHEXT"] = ".test"
 
-            extensioned_file_in_path = os.path.join(self.temp_dir, self.to_text_type("file.test"))
-            open(extensioned_file_in_path, 'w').close()
+            exe = os.path.join(self.temp_dir, self.to_text_type("test.exe"))
+            open(exe, 'w').close()
+            os.chmod(exe, 0o755)
 
+            # default does not match since .exe is not in PATHEXT
+            self.assertIsNone(shutil.which(self.to_text_type("test.exe")))
 
-            self.assertEqual(shutil.which(self.to_text_type('file'), os.F_OK), extensioned_file_in_path)
+            # but if we don't use os.X_OK we're ok not matching PATHEXT
+            self.assertEqual(shutil.which(self.to_text_type("test.exe"), mode=os.F_OK), exe)
 
     # See GH-109590
     @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
-    def test_extensionless_file_resolution_dot_in_pathext(self):
+    def test_pathext_given_extension_preferred(self):
         with os_helper.EnvironmentVarGuard() as env:
-            env['PATHEXT'] = ".test;.;"
-            env['PATH'] = self.temp_dir if isinstance(self.temp_dir, str) else self.temp_dir.decode()
+            env["PATH"] = self.temp_dir if isinstance(self.temp_dir, str) else self.temp_dir.decode()
+            env["PATHEXT"] = ".exe2;.exe"
+
+            exe = os.path.join(self.temp_dir, self.to_text_type("test.exe"))
+            open(exe, 'w').close()
+            os.chmod(exe, 0o755)
 
-            extensionless_file_in_path = os.path.join(self.temp_dir, self.to_text_type("file"))
-            open(extensionless_file_in_path, 'w').close()
+            exe2 = os.path.join(self.temp_dir, self.to_text_type("test.exe2"))
+            open(exe2, 'w').close()
+            os.chmod(exe2, 0o755)
 
-            extensioned_file_in_path = os.path.join(self.temp_dir, self.to_text_type("file.test"))
-            open(extensioned_file_in_path, 'w').close()
+            # even though .exe2 is preferred in PATHEXT, we matched directly to test.exe
+            self.assertEqual(shutil.which(self.to_text_type("test.exe")), exe)
 
-            self.assertEqual(shutil.which(self.to_text_type('file')), extensionless_file_in_path)
+            self.assertEqual(shutil.which(self.to_text_type("test")), exe2)
 
 
 class TestWhichBytes(TestWhich):