path.local: as_cwd: do not chdir to None [ci skip]

CHANGELOG

@@ -4,6 +4,9 @@
 - add ``"importlib"`` pyimport mode for python3.5+, allowing unimportable test suites
   to contain identically named modules.
 
+- fix ``LocalPath.as_cwd()`` not calling ``os.chdir()`` with ``None``, when
+  being invoked from a non-existing directory.
+
 1.7.0 (2018-10-11)
 ==================
 

py/_path/local.py

@@ -579,14 +579,17 @@ def chdir(self):
 
     @contextmanager
     def as_cwd(self):
-        """ return context manager which changes to current dir during the
-        managed "with" context. On __enter__ it returns the old dir.
+        """
+        Return a context manager, which changes to the path's dir during the
+        managed "with" context.
+        On __enter__ it returns the old dir, which might be ``None``.
         """
         old = self.chdir()
         try:
             yield old
         finally:
-            old.chdir()
+            if old is not None:
+                old.chdir()
 
     def realpath(self):
         """ return a new path which contains no symbolic links."""

testing/path/test_local.py

@@ -125,6 +125,19 @@ def test_chdir_gone(self, path1):
         assert path1.chdir() is None
         assert os.getcwd() == str(path1)
 
+        with pytest.raises(py.error.ENOENT):
+            with p.as_cwd():
+                raise NotImplementedError
+
+    @skiponwin32
+    def test_chdir_gone_in_as_cwd(self, path1):
+        p = path1.ensure("dir_to_be_removed", dir=1)
+        p.chdir()
+        p.remove()
+
+        with path1.as_cwd() as old:
+            assert old is None
+
     def test_as_cwd(self, path1):
         dir = path1.ensure("subdir", dir=1)
         old = py.path.local()