bpo-40592: shutil.which will not return None anymore if ; is the last char in PATHEXT

Lib/shutil.py

@@ -1415,7 +1415,7 @@ def which(cmd, mode=os.F_OK | os.X_OK, path=None):
             path.insert(0, curdir)
 
         # PATHEXT is necessary to check on Windows.
-        pathext = os.environ.get("PATHEXT", "").split(os.pathsep)
+        pathext = [ext for ext in os.environ.get("PATHEXT", "").split(os.pathsep) if ext]
         if use_bytes:
             pathext = [os.fsencode(ext) for ext in pathext]
         # See if the given file matches any of the expected path extensions.

Lib/test/test_shutil.py

@@ -1848,6 +1848,23 @@ def test_pathext(self):
             rv = shutil.which(program, path=self.temp_dir)
             self.assertEqual(rv, temp_filexyz.name)
 
+    @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
+    def test_pathext_with_empty_str(self):
+        ext = ".xyz"
+        temp_filexyz = tempfile.NamedTemporaryFile(dir=self.temp_dir,
+                                                   prefix="Tmp2", suffix=ext)
+        os.chmod(temp_filexyz.name, stat.S_IXUSR)
+        self.addCleanup(temp_filexyz.close)
+
+        # strip path and extension
+        program = os.path.basename(temp_filexyz.name)
+        program = os.path.splitext(program)[0]
+
+        with support.EnvironmentVarGuard() as env:
+            env['PATHEXT'] = f"{ext};"  # note the ;
+            rv = shutil.which(program, path=self.temp_dir)
+            self.assertEqual(rv, temp_filexyz.name)
+
 
 class TestWhichBytes(TestWhich):
     def setUp(self):

Misc/NEWS.d/next/Library/2020-05-14-16-01-34.bpo-40592.Cmk855.rst

@@ -0,0 +1 @@
+shutil.which returns not None anymore with ; at the end of PATHEXT