Fix path splitting functions for the simulated file system

CHANGES.md

@@ -23,6 +23,9 @@ The released versions correspond to PyPI releases.
 * fixed `os.rename` failing for the root user in a directory owned by
   another user
   (see [#1341](https://github.com/pytest-dev/pyfakefs/issues/1341))
+* fixed `os.path.basename`, `os.path.splitext` and `os.path.commonpath`
+  not using the path semantics of the simulated file system
+  (see [#1348](https://github.com/pytest-dev/pyfakefs/issues/1348))
 
 ## [Version 6.2.0](https://pypi.python.org/pypi/pyfakefs/6.2.0) (2026-04-12)
 

pyfakefs/fake_path.py

@@ -19,7 +19,9 @@
 import errno
 import functools
 import inspect
+import ntpath
 import os
+import posixpath
 import sys
 from collections.abc import Callable
 from stat import (
@@ -79,6 +81,8 @@ def dir() -> list[str]:
         """
         dir_list = [
             "abspath",
+            "basename",
+            "commonpath",
             "dirname",
             "exists",
             "expanduser",
@@ -99,6 +103,7 @@ def dir() -> list[str]:
             "relpath",
             "split",
             "splitdrive",
+            "splitext",
             "samefile",
         ]
         if sys.version_info >= (3, 12):
@@ -316,6 +321,22 @@ def splitdrive(self, path: AnyStr) -> tuple[AnyStr, AnyStr]:
         supported."""
         return self.filesystem.splitdrive(path)
 
+    def _path_module(self) -> Any:
+        """Return the path module matching the simulated file system."""
+        return ntpath if self.filesystem.is_windows_fs else posixpath
+
+    def basename(self, path: AnyStr) -> AnyStr:
+        """Returns the final component of a pathname."""
+        return self._path_module().basename(path)
+
+    def splitext(self, path: AnyStr) -> tuple[AnyStr, AnyStr]:
+        """Split the path into the file name and the file extension."""
+        return self._path_module().splitext(path)
+
+    def commonpath(self, paths: list[AnyStr]) -> AnyStr:
+        """Returns the longest common sub-path of the given paths."""
+        return self._path_module().commonpath(paths)
+
     def normpath(self, path: AnyStr) -> AnyStr:
         """Normalize path, eliminating double slashes, etc."""
         return self.filesystem.normpath(path)

pyfakefs/tests/fake_filesystem_test.py

@@ -1154,6 +1154,40 @@ def test_dirname(self):
         dirname = "foo!bar"
         self.assertEqual(dirname, self.path.dirname(f"{dirname}!baz"))
 
+    def test_basename_windows(self):
+        # regression test for #1348: backslashes are path separators
+        self.filesystem.is_windows_fs = True
+        self.assertEqual("t.x", self.path.basename(r"C:\a/foo\t.x"))
+        self.assertEqual("t.x", self.path.basename(r"C:\a\foo\t.x"))
+        self.assertEqual(b"t.x", self.path.basename(rb"C:\a\foo\t.x"))
+
+    def test_basename_posix(self):
+        self.filesystem.is_windows_fs = False
+        self.assertEqual(r"foo\t.x", self.path.basename(r"C:\a/foo\t.x"))
+        self.assertEqual(r"C:\a\foo\t.x", self.path.basename(r"C:\a\foo\t.x"))
+
+    def test_splitext_windows(self):
+        # regression test for #1348: a dot after a separator is not an extension
+        self.filesystem.is_windows_fs = True
+        self.assertEqual((r"C:\a.b\foo", ""), self.path.splitext(r"C:\a.b\foo"))
+        self.assertEqual((r"C:\a\foo", ".txt"), self.path.splitext(r"C:\a\foo.txt"))
+        self.assertEqual((rb"C:\a.b\foo", b""), self.path.splitext(rb"C:\a.b\foo"))
+
+    def test_splitext_posix(self):
+        self.filesystem.is_windows_fs = False
+        self.assertEqual((r"C:\a", r".b\foo"), self.path.splitext(r"C:\a.b\foo"))
+
+    def test_commonpath_windows(self):
+        # regression test for #1348: mixed separators in the same drive
+        self.filesystem.is_windows_fs = True
+        self.assertEqual(r"C:\a", self.path.commonpath([r"C:\a\b", r"C:/a\c"]))
+        self.assertEqual(r"C:\a", self.path.commonpath([r"C:\a\b", r"C:\a\c"]))
+        self.assertEqual(rb"C:\a", self.path.commonpath([rb"C:\a\b", rb"C:\a\c"]))
+
+    def test_commonpath_posix(self):
+        self.filesystem.is_windows_fs = False
+        self.assertEqual("", self.path.commonpath([r"C:\a\b", r"C:/a\c"]))
+
     def test_join_strings(self):
         components = ["foo", "bar", "baz"]
         self.assertEqual("foo!bar!baz", self.path.join(*components))