Correctly handle alternative path separators in path resolving

- fixes pytest-dev#530

CHANGES.md

@@ -8,7 +8,9 @@ The released versions correspond to PyPi releases.
     (see [#526](../../issues/526))
   * Make sure filesystem modules in `pathlib` are patched
     (see [#527](../../issues/527))
-
+  * Make sure that alternative path separators are correctly handled under Windows
+    (see [#530](../../issues/530))
+
 #### Infrastructure
   * Make sure all temporary files from real fs tests are removed
 

pyfakefs/fake_filesystem.py

@@ -977,6 +977,13 @@ def _alternative_path_separator(self, path):
         """Return the alternative path separator as the same type as path"""
         return self._matching_string(path, self.alternative_path_separator)
 
+    def _starts_with_sep(self, path):
+        """Return True if path starts with a path separator."""
+        sep = self._path_separator(path)
+        altsep = self._alternative_path_separator(path)
+        return (path.startswith(sep) or altsep is not None and
+                path.startswith(altsep))
+
     def add_mount_point(self, path, total_size=None):
         """Add a new mount point for a filesystem device.
         The mount point gets a new unique device number.
@@ -1409,7 +1416,8 @@ def components_to_path():
                     path_components[len(normalized_components):])
             sep = self._path_separator(path)
             normalized_path = sep.join(normalized_components)
-            if path.startswith(sep) and not normalized_path.startswith(sep):
+            if (self._starts_with_sep(path) and not
+                    self._starts_with_sep(normalized_path)):
                 normalized_path = sep + normalized_path
             return normalized_path
 
@@ -3142,13 +3150,7 @@ def isabs(self, path):
         if self.filesystem.is_windows_fs:
             path = self.splitdrive(path)[1]
         path = make_string_path(path)
-        sep = self.filesystem._path_separator(path)
-        altsep = self.filesystem._alternative_path_separator(path)
-        if self.filesystem.is_windows_fs:
-            return len(path) > 0 and path[:1] in (sep, altsep)
-        else:
-            return (path.startswith(sep) or
-                    altsep is not None and path.startswith(altsep))
+        return self.filesystem._starts_with_sep(path)
 
     def isdir(self, path):
         """Determine if path identifies a directory."""
@@ -3244,13 +3246,10 @@ def getcwd():
                 return self.os.getcwd()
 
         path = make_string_path(path)
-        sep = self.filesystem._path_separator(path)
-        altsep = self.filesystem._alternative_path_separator(path)
         if not self.isabs(path):
             path = self.join(getcwd(), path)
         elif (self.filesystem.is_windows_fs and
-              path.startswith(sep) or altsep is not None and
-              path.startswith(altsep)):
+              self.filesystem._starts_with_sep(path)):
             cwd = getcwd()
             if self.filesystem._starts_with_drive_letter(cwd):
                 path = self.join(cwd[:2], path)

pyfakefs/pytest_tests/pytest_fixture_param_test.py

@@ -12,6 +12,7 @@
 
 # Example for a test using a custom pytest fixture with an argument to Patcher
 # Needs Python >= 3.6
+import os
 
 import pytest
 
@@ -41,3 +42,11 @@ def check_that_example_file_is_in_fake_fs():
         assert file.read() == 'stuff here'
     assert example.EXAMPLE_FILE.read_text() == 'stuff here'
     assert example.EXAMPLE_FILE.is_file()
+
+
+def test_twice_chdir(fs):
+    # regression test for #530 - make sure that
+    # alternative path separators are correctly handled under Windows
+    fs.create_dir("/absolute/path/to/directory")
+    os.chdir("/absolute/path/to/directory")
+    os.chdir("/absolute/path/to/directory")