GH-75586: Make shutil.which() on Windows more consistent with the OS (GH-103179)

7 files changed, 233 insertions, 54 deletions

diff --git a/Doc/library/shutil.rst b/Doc/library/shutil.rst
index acba662..373cc7d 100644
--- a/Doc/library/shutil.rst
+++ b/Doc/library/shutil.rst
@@ -433,23 +433,43 @@ Directory and files operations
    When no *path* is specified, the results of :func:`os.environ` are used,
    returning either the "PATH" value or a fallback of :attr:`os.defpath`.
 
-   On Windows, the current directory is always prepended to the *path* whether
-   or not you use the default or provide your own, which is the behavior the
-   command shell uses when finding executables.  Additionally, when finding the
-   *cmd* in the *path*, the ``PATHEXT`` environment variable is checked.  For
-   example, if you call ``shutil.which("python")``, :func:`which` will search
-   ``PATHEXT`` to know that it should look for ``python.exe`` within the *path*
-   directories.  For example, on Windows::
+   On Windows, the current directory is prepended to the *path* if *mode* does
+   not include ``os.X_OK``. When the *mode* does include ``os.X_OK``, the
+   Windows API ``NeedCurrentDirectoryForExePathW`` will be consulted to
+   determine if the current directory should be prepended to *path*. To avoid
+   consulting the current working directory for executables: set the environment
+   variable ``NoDefaultCurrentDirectoryInExePath``.
+
+   Also on Windows, the ``PATHEXT`` variable is used to resolve commands
+   that may not already include an extension. For example, if you call
+   ``shutil.which("python")``, :func:`which` will search ``PATHEXT``
+   to know that it should look for ``python.exe`` within the *path*
+   directories. For example, on Windows::
 
       >>> shutil.which("python")
       'C:\\Python33\\python.EXE'
 
+   This is also applied when *cmd* is a path that contains a directory
+   component::
+
+      >> shutil.which("C:\\Python33\\python")
+      'C:\\Python33\\python.EXE'
+
    .. versionadded:: 3.3
 
    .. versionchanged:: 3.8
       The :class:`bytes` type is now accepted.  If *cmd* type is
       :class:`bytes`, the result type is also :class:`bytes`.
 
+   .. versionchanged:: 3.12
+      On Windows, the current directory is no longer prepended to the search
+      path if *mode* includes ``os.X_OK`` and WinAPI
+      ``NeedCurrentDirectoryForExePathW(cmd)`` is false, else the current
+      directory is prepended even if it is already in the search path;
+      ``PATHEXT`` is used now even when *cmd* includes a directory component
+      or ends with an extension that is in ``PATHEXT``; and filenames that
+      have no extension can now be found.
+
 .. exception:: Error
 
    This exception collects exceptions that are raised during a multi-file
diff --git a/Doc/whatsnew/3.12.rst b/Doc/whatsnew/3.12.rst
index 88b9982..cc2c056 100644
--- a/Doc/whatsnew/3.12.rst
+++ b/Doc/whatsnew/3.12.rst
@@ -343,6 +343,20 @@ shutil
   will be removed in Python 3.14.
   (Contributed by Irit Katriel in :gh:`102828`.)
 
+* :func:`shutil.which` now consults the *PATHEXT* environment variable to
+  find matches within *PATH* on Windows even when the given *cmd* includes
+  a directory component.
+  (Contributed by Charles Machalow in :gh:`103179`.)
+
+  :func:`shutil.which` will call ``NeedCurrentDirectoryForExePathW`` when
+  querying for executables on Windows to determine if the current working
+  directory should be prepended to the search path.
+  (Contributed by Charles Machalow in :gh:`103179`.)
+
+  :func:`shutil.which` will return a path matching the *cmd* with a component
+  from ``PATHEXT`` prior to a direct match elsewhere in the search path on
+  Windows.
+  (Contributed by Charles Machalow in :gh:`103179`.)
 
 sqlite3
 -------
diff --git a/Lib/shutil.py b/Lib/shutil.py
index b057640..8b37864 100644
--- a/Lib/shutil.py
+++ b/Lib/shutil.py
@@ -40,6 +40,9 @@ if os.name == 'posix':
 elif _WINDOWS:
     import nt
 
+if sys.platform == 'win32':
+    import _winapi
+
 COPY_BUFSIZE = 1024 * 1024 if _WINDOWS else 64 * 1024
 # This should never be removed, see rationale in:
 # https://bugs.python.org/issue43743#msg393429
@@ -1449,6 +1452,16 @@ def _access_check(fn, mode):
             and not os.path.isdir(fn))
 
 
+def _win_path_needs_curdir(cmd, mode):
+    """
+    On Windows, we can use NeedCurrentDirectoryForExePath to figure out
+    if we should add the cwd to PATH when searching for executables if
+    the mode is executable.
+    """
+    return (not (mode & os.X_OK)) or _winapi.NeedCurrentDirectoryForExePath(
+                os.fsdecode(cmd))
+
+
 def which(cmd, mode=os.F_OK | os.X_OK, path=None):
     """Given a command, mode, and a PATH string, return the path which
     conforms to the given mode on the PATH, or None if there is no such
@@ -1459,60 +1472,54 @@ def which(cmd, mode=os.F_OK | os.X_OK, path=None):
     path.
 
     """
-    # If we're given a path with a directory part, look it up directly rather
-    # than referring to PATH directories. This includes checking relative to the
-    # current directory, e.g. ./script
-    if os.path.dirname(cmd):
-        if _access_check(cmd, mode):
-            return cmd
-        return None
-
     use_bytes = isinstance(cmd, bytes)
 
-    if path is None:
-        path = os.environ.get("PATH", None)
-        if path is None:
-            try:
-                path = os.confstr("CS_PATH")
-            except (AttributeError, ValueError):
-                # os.confstr() or CS_PATH is not available
-                path = os.defpath
-        # bpo-35755: Don't use os.defpath if the PATH environment variable is
-        # set to an empty string
-
-    # PATH='' doesn't match, whereas PATH=':' looks in the current directory
-    if not path:
-        return None
-
-    if use_bytes:
-        path = os.fsencode(path)
-        path = path.split(os.fsencode(os.pathsep))
+    # If we're given a path with a directory part, look it up directly rather
+    # than referring to PATH directories. This includes checking relative to
+    # the current directory, e.g. ./script
+    dirname, cmd = os.path.split(cmd)
+    if dirname:
+        path = [dirname]
     else:
-        path = os.fsdecode(path)
-        path = path.split(os.pathsep)
+        if path is None:
+            path = os.environ.get("PATH", None)
+            if path is None:
+                try:
+                    path = os.confstr("CS_PATH")
+                except (AttributeError, ValueError):
+                    # os.confstr() or CS_PATH is not available
+                    path = os.defpath
+            # bpo-35755: Don't use os.defpath if the PATH environment variable
+            # is set to an empty string
+
+        # PATH='' doesn't match, whereas PATH=':' looks in the current
+        # directory
+        if not path:
+            return None
 
-    if sys.platform == "win32":
-        # The current directory takes precedence on Windows.
-        curdir = os.curdir
         if use_bytes:
-            curdir = os.fsencode(curdir)
-        if curdir not in path:
+            path = os.fsencode(path)
+            path = path.split(os.fsencode(os.pathsep))
+        else:
+            path = os.fsdecode(path)
+            path = path.split(os.pathsep)
+
+        if sys.platform == "win32" and _win_path_needs_curdir(cmd, mode):
+            curdir = os.curdir
+            if use_bytes:
+                curdir = os.fsencode(curdir)
             path.insert(0, curdir)
 
+    if sys.platform == "win32":
         # PATHEXT is necessary to check on Windows.
         pathext_source = os.getenv("PATHEXT") or _WIN_DEFAULT_PATHEXT
         pathext = [ext for ext in pathext_source.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.
-        # This will allow us to short circuit when given "python.exe".
-        # If it does match, only test that one, otherwise we have to try
-        # others.
-        if any(cmd.lower().endswith(ext.lower()) for ext in pathext):
-            files = [cmd]
-        else:
-            files = [cmd + ext for ext in pathext]
+
+        # Always try checking the originally given cmd, if it doesn't match, try pathext
+        files = [cmd] + [cmd + ext for ext in pathext]
     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.
diff --git a/Lib/test/test_shutil.py b/Lib/test/test_shutil.py
index 1c0589c..9eaf167 100644
--- a/Lib/test/test_shutil.py
+++ b/Lib/test/test_shutil.py
@@ -2034,18 +2034,68 @@ class TestWhich(BaseTest, unittest.TestCase):
             rv = shutil.which(relpath, path=base_dir)
             self.assertIsNone(rv)
 
-    def test_cwd(self):
+    @unittest.skipUnless(sys.platform != "win32",
+                         "test is for non win32")
+    def test_cwd_non_win32(self):
         # Issue #16957
         base_dir = os.path.dirname(self.dir)
         with os_helper.change_cwd(path=self.dir):
             rv = shutil.which(self.file, path=base_dir)
-            if sys.platform == "win32":
-                # Windows: current directory implicitly on PATH
+            # non-win32: shouldn't match in the current directory.
+            self.assertIsNone(rv)
+
+    @unittest.skipUnless(sys.platform == "win32",
+                         "test is for win32")
+    def test_cwd_win32(self):
+        base_dir = os.path.dirname(self.dir)
+        with os_helper.change_cwd(path=self.dir):
+            with unittest.mock.patch('shutil._win_path_needs_curdir', return_value=True):
+                rv = shutil.which(self.file, path=base_dir)
+                # Current directory implicitly on PATH
                 self.assertEqual(rv, os.path.join(self.curdir, self.file))
-            else:
-                # Other platforms: shouldn't match in the current directory.
+            with unittest.mock.patch('shutil._win_path_needs_curdir', return_value=False):
+                rv = shutil.which(self.file, path=base_dir)
+                # Current directory not on PATH
                 self.assertIsNone(rv)
 
+    @unittest.skipUnless(sys.platform == "win32",
+                         "test is for win32")
+    def test_cwd_win32_added_before_all_other_path(self):
+        base_dir = pathlib.Path(os.fsdecode(self.dir))
+
+        elsewhere_in_path_dir = base_dir / 'dir1'
+        elsewhere_in_path_dir.mkdir()
+        match_elsewhere_in_path = elsewhere_in_path_dir / 'hello.exe'
+        match_elsewhere_in_path.touch()
+
+        exe_in_cwd = base_dir / 'hello.exe'
+        exe_in_cwd.touch()
+
+        with os_helper.change_cwd(path=base_dir):
+            with unittest.mock.patch('shutil._win_path_needs_curdir', return_value=True):
+                rv = shutil.which('hello.exe', path=elsewhere_in_path_dir)
+
+            self.assertEqual(os.path.abspath(rv), os.path.abspath(exe_in_cwd))
+
+    @unittest.skipUnless(sys.platform == "win32",
+                         "test is for win32")
+    def test_pathext_match_before_path_full_match(self):
+        base_dir = pathlib.Path(os.fsdecode(self.dir))
+        dir1 = base_dir / 'dir1'
+        dir2 = base_dir / 'dir2'
+        dir1.mkdir()
+        dir2.mkdir()
+
+        pathext_match = dir1 / 'hello.com.exe'
+        path_match = dir2 / 'hello.com'
+        pathext_match.touch()
+        path_match.touch()
+
+        test_path = os.pathsep.join([str(dir1), str(dir2)])
+        assert os.path.basename(shutil.which(
+            'hello.com', path=test_path, mode = os.F_OK
+        )).lower() == 'hello.com.exe'
+
     @os_helper.skip_if_dac_override
     def test_non_matching_mode(self):
         # Set the file read-only and ask for writeable files.
@@ -2179,6 +2229,32 @@ class TestWhich(BaseTest, unittest.TestCase):
             rv = shutil.which(program, path=self.temp_dir)
             self.assertEqual(rv, temp_filexyz.name)
 
+    # See GH-75586
+    @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
+    def test_pathext_applied_on_files_in_path(self):
+        with os_helper.EnvironmentVarGuard() as env:
+            env["PATH"] = self.temp_dir
+            env["PATHEXT"] = ".test"
+
+            test_path = pathlib.Path(self.temp_dir) / "test_program.test"
+            test_path.touch(mode=0o755)
+
+            self.assertEqual(shutil.which("test_program"), str(test_path))
+
+    # See GH-75586
+    @unittest.skipUnless(sys.platform == "win32", 'test specific to Windows')
+    def test_win_path_needs_curdir(self):
+        with unittest.mock.patch('_winapi.NeedCurrentDirectoryForExePath', return_value=True) as need_curdir_mock:
+            self.assertTrue(shutil._win_path_needs_curdir('dontcare', os.X_OK))
+            need_curdir_mock.assert_called_once_with('dontcare')
+            need_curdir_mock.reset_mock()
+            self.assertTrue(shutil._win_path_needs_curdir('dontcare', 0))
+            need_curdir_mock.assert_not_called()
+
+        with unittest.mock.patch('_winapi.NeedCurrentDirectoryForExePath', return_value=False) as need_curdir_mock:
+            self.assertFalse(shutil._win_path_needs_curdir('dontcare', os.X_OK))
+            need_curdir_mock.assert_called_once_with('dontcare')
+
 
 class TestWhichBytes(TestWhich):
     def setUp(self):
diff --git a/Misc/NEWS.d/next/Library/2023-04-02-22-04-26.gh-issue-75586.526iJm.rst b/Misc/NEWS.d/next/Library/2023-04-02-22-04-26.gh-issue-75586.526iJm.rst
new file mode 100644
index 0000000..8ec568e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-04-02-22-04-26.gh-issue-75586.526iJm.rst
@@ -0,0 +1 @@
+Fix various Windows-specific issues with ``shutil.which``.
diff --git a/Modules/_winapi.c b/Modules/_winapi.c
index 83cde75..fa380b8 100644
--- a/Modules/_winapi.c
+++ b/Modules/_winapi.c
@@ -2054,6 +2054,26 @@ _winapi__mimetypes_read_windows_registry_impl(PyObject *module,
 #undef CB_TYPE
 }
 
+/*[clinic input]
+_winapi.NeedCurrentDirectoryForExePath -> bool
+
+    exe_name: LPCWSTR
+    /
+[clinic start generated code]*/
+
+static int
+_winapi_NeedCurrentDirectoryForExePath_impl(PyObject *module,
+                                            LPCWSTR exe_name)
+/*[clinic end generated code: output=a65ec879502b58fc input=972aac88a1ec2f00]*/
+{
+    BOOL result;
+
+    Py_BEGIN_ALLOW_THREADS
+    result = NeedCurrentDirectoryForExePathW(exe_name);
+    Py_END_ALLOW_THREADS
+
+    return result;
+}
 
 static PyMethodDef winapi_functions[] = {
     _WINAPI_CLOSEHANDLE_METHODDEF
@@ -2089,6 +2109,7 @@ static PyMethodDef winapi_functions[] = {
     _WINAPI_GETACP_METHODDEF
     _WINAPI_GETFILETYPE_METHODDEF
     _WINAPI__MIMETYPES_READ_WINDOWS_REGISTRY_METHODDEF
+    _WINAPI_NEEDCURRENTDIRECTORYFOREXEPATH_METHODDEF
     {NULL, NULL}
 };
 
diff --git a/Modules/clinic/_winapi.c.h b/Modules/clinic/_winapi.c.h
index 891b3f8..7bc63e6 100644
--- a/Modules/clinic/_winapi.c.h
+++ b/Modules/clinic/_winapi.c.h
@@ -1371,4 +1371,44 @@ _winapi__mimetypes_read_windows_registry(PyObject *module, PyObject *const *args
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=edb1a9d1bbfd6394 input=a9049054013a1b77]*/
+
+PyDoc_STRVAR(_winapi_NeedCurrentDirectoryForExePath__doc__,
+"NeedCurrentDirectoryForExePath($module, exe_name, /)\n"
+"--\n"
+"\n");
+
+#define _WINAPI_NEEDCURRENTDIRECTORYFOREXEPATH_METHODDEF    \
+    {"NeedCurrentDirectoryForExePath", (PyCFunction)_winapi_NeedCurrentDirectoryForExePath, METH_O, _winapi_NeedCurrentDirectoryForExePath__doc__},
+
+static int
+_winapi_NeedCurrentDirectoryForExePath_impl(PyObject *module,
+                                            LPCWSTR exe_name);
+
+static PyObject *
+_winapi_NeedCurrentDirectoryForExePath(PyObject *module, PyObject *arg)
+{
+    PyObject *return_value = NULL;
+    LPCWSTR exe_name = NULL;
+    int _return_value;
+
+    if (!PyUnicode_Check(arg)) {
+        _PyArg_BadArgument("NeedCurrentDirectoryForExePath", "argument", "str", arg);
+        goto exit;
+    }
+    exe_name = PyUnicode_AsWideCharString(arg, NULL);
+    if (exe_name == NULL) {
+        goto exit;
+    }
+    _return_value = _winapi_NeedCurrentDirectoryForExePath_impl(module, exe_name);
+    if ((_return_value == -1) && PyErr_Occurred()) {
+        goto exit;
+    }
+    return_value = PyBool_FromLong((long)_return_value);
+
+exit:
+    /* Cleanup for exe_name */
+    PyMem_Free((void *)exe_name);
+
+    return return_value;
+}
+/*[clinic end generated code: output=96ea65ece7912d0a input=a9049054013a1b77]*/