Add FakeFilesystem.os for more convenient OS setting

- the setter changes all needed attributes and resets the file system
- reset now also resets pathlib to adapt the path separators
- adapt documentation

Author: mrbean-bremen

CHANGES.md

@@ -9,6 +9,7 @@ The released versions correspond to PyPi releases.
   * add possibility to patch `io.open_code` using the new argument 
     `patch_open_code` (since Python 3.8)
     (see [#554](../../issues/554))
+  * add possibility to set file system OS via `FakeFilesystem.os` 
 
 #### Fixes
   * fix handling of real files in combination with `home` if simulating

docs/usage.rst

@@ -675,8 +675,15 @@ Simulating other file systems
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Pyfakefs supports Linux, MacOS and Windows operating systems. By default,
 the file system of the OS where the tests run is assumed, but it is possible
-to simulate other file systems to an extent. ``pyfakefs.FakeFilesystem`` has
-a few attributes that can be changed to simulate another file system:
+to simulate other file systems to some extent. To set a specific file
+system, you can change ``pyfakefs.FakeFilesystem.os`` to one of
+``OSType.LINUX``, ``OSType.MACOS`` and ``OSType.WINDOWS``. On doing so, the
+behavior of pyfakefs is adapted to the respective file system. Note that
+setting this causes the fake file system to be reset, so you should call it
+before adding any files.
+
+Setting the ``os`` attributes changes a number of ``pyfakefs.FakeFilesystem``
+attributes, which can also be set separately if needed:
 
   - ``is windows_fs`` -  if ``True`` a Windows file system (NTFS) is assumed
   - ``is_macos`` - if ``True`` and ``is windows_fs`` is ``False``, the
@@ -694,11 +701,13 @@ The following test works both under Windows and Linux:
 
 .. code:: python
 
-  def test_windows_paths(fs):
-      fs.is_windows_fs = True
-      path = "C:/foo/bar"
-      assert os.path.splitdrive(path) == ("C:", "/foo/bar")
+  from pyfakefs.fake_filesystem import OSType
 
+  def test_windows_paths(fs):
+      fs.os = OSType.WINDOWS
+      assert r"C:\foo\bar" == os.path.join('C:\\', 'foo', 'bar'))
+      assert os.path.splitdrive(r"C:\foo\bar") == ("C:", r"\foo\bar")
+      assert os.path.ismount("C:")
 
 Troubleshooting
 ---------------

pyfakefs/fake_filesystem.py

@@ -158,6 +158,13 @@
 GROUP_ID = 1 if IS_WIN else os.getgid()
 
 
+class OSType(Enum):
+    """Defines the real or simulated OS of the underlying file system."""
+    LINUX = "linux"
+    MACOS = "macos"
+    WINDOWS = "windows"
+
+
 class PatchMode(Enum):
     """Defines if patching shall be on, off, or in automatic mode.
     Currently only used for `patch_open_code` option.
@@ -905,6 +912,24 @@ def __init__(self, path_separator=os.path.sep, total_size=None,
     def is_linux(self):
         return not self.is_windows_fs and not self.is_macos
 
+    @property
+    def os(self):
+        """Return the real or simulated type of operating system."""
+        return (OSType.WINDOWS if self.is_windows_fs else
+                OSType.MACOS if self.is_macos else OSType.LINUX)
+
+    @os.setter
+    def os(self, value):
+        """Set the simulated type of operating system underlying the fake
+        file system."""
+        self.is_windows_fs = value == OSType.WINDOWS
+        self.is_macos = value == OSType.MACOS
+        self.is_case_sensitive = value == OSType.LINUX
+        self.path_separator = '\\' if value == OSType.WINDOWS else '/'
+        self.alternative_path_separator = ('/' if value == OSType.WINDOWS
+                                           else None)
+        self.reset()
+
     def reset(self, total_size=None):
         """Remove all file system contents and reset the root."""
         self.root = FakeDirectory(self.path_separator, filesystem=self)
@@ -917,6 +942,8 @@ def reset(self, total_size=None):
         self.mount_points = {}
         self.add_mount_point(self.root.name, total_size)
         self._add_standard_streams()
+        from pyfakefs import fake_pathlib
+        fake_pathlib.init_module(self)
 
     def pause(self):
         """Pause the patching of the file system modules until `resume` is

pyfakefs/tests/fake_filesystem_unittest_test.py

@@ -34,6 +34,7 @@
 import pyfakefs.tests.import_as_example
 from pyfakefs import fake_filesystem_unittest, fake_filesystem
 from pyfakefs.extra_packages import pathlib
+from pyfakefs.fake_filesystem import OSType
 from pyfakefs.fake_filesystem_unittest import (
     Patcher, Pause, patchfs, PatchMode
 )
@@ -747,10 +748,10 @@ def test_run_module(self):
 class TestOtherFS(fake_filesystem_unittest.TestCase):
     def setUp(self):
         self.setUpPyfakefs()
-        self.fs.is_windows_fs = os.name != 'nt'
 
     def test_real_file_with_home(self):
         """Regression test for #558"""
+        self.fs.is_windows_fs = os.name != 'nt'
         self.fs.add_real_file(__file__)
         with open(__file__) as f:
             self.assertTrue(f.read())
@@ -762,6 +763,39 @@ def test_real_file_with_home(self):
         with open(__file__) as f:
             self.assertTrue(f.read())
 
+    def test_windows(self):
+        self.fs.os = OSType.WINDOWS
+        path = r'C:\foo\bar'
+        self.assertEqual(path, os.path.join('C:\\', 'foo', 'bar'))
+        self.assertEqual(('C:', r'\foo\bar'), os.path.splitdrive(path))
+        self.fs.create_file(path)
+        self.assertTrue(os.path.exists(path))
+        self.assertTrue(os.path.exists(path.upper()))
+        self.assertTrue(os.path.ismount(r'\\share\foo'))
+        self.assertTrue(os.path.ismount(r'C:'))
+
+    def test_linux(self):
+        self.fs.os = OSType.LINUX
+        path = '/foo/bar'
+        self.assertEqual(path, os.path.join('/', 'foo', 'bar'))
+        self.assertEqual(('', 'C:/foo/bar'), os.path.splitdrive('C:/foo/bar'))
+        self.fs.create_file(path)
+        self.assertTrue(os.path.exists(path))
+        self.assertFalse(os.path.exists(path.upper()))
+        self.assertTrue(os.path.ismount('/'))
+        self.assertFalse(os.path.ismount('//share/foo'))
+
+    def test_macos(self):
+        self.fs.os = OSType.MACOS
+        path = '/foo/bar'
+        self.assertEqual(path, os.path.join('/', 'foo', 'bar'))
+        self.assertEqual(('', 'C:/foo/bar'), os.path.splitdrive('C:/foo/bar'))
+        self.fs.create_file(path)
+        self.assertTrue(os.path.exists(path))
+        self.assertTrue(os.path.exists(path.upper()))
+        self.assertTrue(os.path.ismount('/'))
+        self.assertFalse(os.path.ismount('//share/foo'))
+
 
 if __name__ == "__main__":
     unittest.main()