Added "closed" property to file wrapper

- see #380

CHANGES.md

@@ -10,6 +10,7 @@ The release versions are PyPi releases.
   * use README.md in pypi ([#358](../../issues/358))
 
 #### Fixes
+  * `closed` attribute was not implemented in fake file ([#380](../../issues/380))
   * `add_real_directory` did not behave correctly for nested paths
   * several fixes for the behavior if using file paths that end with a path 
   separator:

pyfakefs/fake_filesystem.py

@@ -4251,6 +4251,11 @@ def close(self):
         if self.delete_on_close:
             self._filesystem.remove_object(self.get_object().path)
 
+    @property
+    def closed(self):
+        """Simulate the `closed` attribute on file."""
+        return not self._is_open()
+
     def flush(self):
         """Flush file contents to 'disk'."""
         self._check_open_file()

pyfakefs/tests/fake_open_test.py

@@ -892,6 +892,17 @@ def test_seek_outside_and_truncate_sets_size_in_append_mode(self):
         # Regression test for #295
         self.check_seek_outside_and_truncate_sets_size('a')
 
+    def test_closed(self):
+        file_path = self.make_path('foo')
+        f = self.open(file_path, 'w')
+        self.assertFalse(f.closed)
+        f.close()
+        self.assertTrue(f.closed)
+        f = self.open(file_path)
+        self.assertFalse(f.closed)
+        f.close()
+        self.assertTrue(f.closed)
+
     def test_closing_closed_file_does_nothing(self):
         # Regression test for #299
         file_path = self.make_path('baz')
@@ -902,6 +913,7 @@ def test_closing_closed_file_does_nothing(self):
             f0.close()
             self.assertEqual('', f1.read())
 
+
     @unittest.skipIf(TestCase.is_python2,
                      'closefd argument not available in Python2')
     def test_closing_file_with_different_close_mode(self):