Fix pure windows path behavior (#1011)

* Hard-code path separators in `Pure*Path` classes
* Fixes #1006

CHANGES.md

@@ -11,6 +11,9 @@ The released versions correspond to PyPI releases.
 * The usage of the `pathlib2` and `scandir` modules in pyfakefs is now deprecated.
   They will now cause deprecation warnings if still used. Support for patching
   these modules will be removed in pyfakefs 6.0.
+* `PureWindowsPath` and `PurePosixPath` now use filesystem-independent path separators,
+  and their path-parsing behaviors are now consistent regardless of runtime platform
+  and/or faked filesystem customization (see [#1006](../../issues/1006)).
 
 ## [Version 5.4.1](https://pypi.python.org/pypi/pyfakefs/5.4.0) (2024-04-11)
 Fixes a regression.

pyfakefs/fake_pathlib.py

@@ -55,15 +55,38 @@ def init_module(filesystem):
     # pylint: disable=protected-access
     FakePath.filesystem = filesystem
     if sys.version_info < (3, 12):
-        FakePathlibModule.PureWindowsPath._flavour = _FakeWindowsFlavour(filesystem)
-        FakePathlibModule.PurePosixPath._flavour = _FakePosixFlavour(filesystem)
+        FakePathlibModule.WindowsPath._flavour = _FakeWindowsFlavour(filesystem)
+        FakePathlibModule.PosixPath._flavour = _FakePosixFlavour(filesystem)
+
+        # Pure POSIX path separators must be filesystem-independent.
+        fake_pure_posix_flavour = _FakePosixFlavour(filesystem)
+        fake_pure_posix_flavour.sep = "/"
+        fake_pure_posix_flavour.altsep = None
+        FakePathlibModule.PurePosixPath._flavour = fake_pure_posix_flavour
+
+        # Pure Windows path separators must be filesystem-independent.
+        fake_pure_nt_flavour = _FakePosixFlavour(filesystem)
+        fake_pure_nt_flavour.sep = "\\"
+        fake_pure_nt_flavour.altsep = "/"
+        FakePathlibModule.PureWindowsPath._flavour = fake_pure_nt_flavour
     else:
         # in Python 3.12, the flavour is no longer an own class,
         # but points to the os-specific path module (posixpath/ntpath)
         fake_os = FakeOsModule(filesystem)
-        fake_path = fake_os.path
-        FakePathlibModule.PureWindowsPath._flavour = fake_path
-        FakePathlibModule.PurePosixPath._flavour = fake_path
+        FakePathlibModule.PosixPath._flavour = fake_os.path
+        FakePathlibModule.WindowsPath._flavour = fake_os.path
+
+        # Pure POSIX path separators must be filesystem independent.
+        fake_pure_posix_os = FakeOsModule(filesystem)
+        fake_pure_posix_os.path.sep = "/"
+        fake_pure_posix_os.path.altsep = None
+        FakePathlibModule.PurePosixPath._flavour = fake_pure_posix_os.path
+
+        # Pure Windows path separators must be filesystem independent.
+        fake_pure_nt_os = FakeOsModule(filesystem)
+        fake_pure_nt_os.path.sep = "\\"
+        fake_pure_nt_os.path.altsep = "/"
+        FakePathlibModule.PureWindowsPath._flavour = fake_pure_nt_os.path
 
 
 def _wrap_strfunc(strfunc):

pyfakefs/tests/fake_pathlib_test.py

@@ -1286,5 +1286,31 @@ def test_is_file_for_unreadable_dir_windows(self):
             path.is_file()
 
 
+class FakePathlibModulePurePathTest(unittest.TestCase):
+    def test_windows_pure_path_parsing(self):
+        """Verify faked pure Windows paths use filesystem-independent separators."""
+
+        path = r"C:\Windows\cmd.exe"
+        self.assertEqual(
+            fake_pathlib.FakePathlibModule.PureWindowsPath(path).stem,
+            pathlib.PureWindowsPath(path).stem,
+        )
+
+        path = r"C:/Windows/cmd.exe"
+        self.assertEqual(
+            fake_pathlib.FakePathlibModule.PureWindowsPath(path).stem,
+            pathlib.PureWindowsPath(path).stem,
+        )
+
+    def test_posix_pure_path_parsing(self):
+        """Verify faked pure POSIX paths use filesystem-independent separators."""
+
+        path = r"/bin/bash"
+        self.assertEqual(
+            fake_pathlib.FakePathlibModule.PurePosixPath(path).stem,
+            pathlib.PurePosixPath(path).stem,
+        )
+
+
 if __name__ == "__main__":
     unittest.main(verbosity=2)