Issue #21750: mock_open.read_data can now be read from each instance, as it

could in Python 3.3.

Lib/unittest/mock.py

@@ -2265,6 +2265,7 @@ def _iterate_read_data(read_data):
     for line in data_as_list:
         yield line
 
+
 def mock_open(mock=None, read_data=''):
     """
     A helper function to create a mock to replace the use of `open`. It works
@@ -2277,6 +2278,16 @@ def mock_open(mock=None, read_data=''):
     `read_data` is a string for the `read` methoddline`, and `readlines` of the
     file handle to return.  This is an empty string by default.
     """
+    global file_spec
+    if file_spec is None:
+        import _io
+        file_spec = list(set(dir(_io.TextIOWrapper)).union(set(dir(_io.BytesIO))))
+
+    if mock is None:
+        mock = MagicMock(name='open', spec=open)
+
+    def make_handle(*args, **kwargs):
+        # Arg checking is handled by __call__
         def _readlines_side_effect(*args, **kwargs):
             if handle.readlines.return_value is not None:
                 return handle.readlines.return_value
@@ -2294,15 +2305,6 @@ def mock_open(mock=None, read_data=''):
             for line in _data:
                 yield line
 
-
-    global file_spec
-    if file_spec is None:
-        import _io
-        file_spec = list(set(dir(_io.TextIOWrapper)).union(set(dir(_io.BytesIO))))
-
-    if mock is None:
-        mock = MagicMock(name='open', spec=open)
-
         handle = MagicMock(spec=file_spec)
         handle.__enter__.return_value = handle
 
@@ -2316,8 +2318,10 @@ def mock_open(mock=None, read_data=''):
         handle.read.side_effect = _read_side_effect
         handle.readline.side_effect = _readline_side_effect()
         handle.readlines.side_effect = _readlines_side_effect
+        _check_and_set_parent(mock, handle, None, '()')
+        return handle
 
-    mock.return_value = handle
+    mock.side_effect = make_handle
     return mock
 
 

Lib/unittest/test/testmock/testmock.py

@@ -1326,6 +1326,11 @@ class MockTest(unittest.TestCase):
             self.assertEqual(m.mock_calls, [call.__int__(), call.__float__()])
             self.assertEqual(m.method_calls, [])
 
+    def test_mock_open_reuse_issue_21750(self):
+        mocked_open = mock.mock_open(read_data='data')
+        f1 = mocked_open('a-name')
+        f2 = mocked_open('another-name')
+        self.assertEqual(f1.read(), f2.read())
 
     def test_mock_parents(self):
         for Klass in Mock, MagicMock:

Lib/unittest/test/testmock/testwith.py

@@ -141,7 +141,6 @@ class TestMockOpen(unittest.TestCase):
 
     def test_mock_open_context_manager(self):
         mock = mock_open()
-        handle = mock.return_value
         with patch('%s.open' % __name__, mock, create=True):
             with open('foo') as f:
                 f.read()
@@ -149,8 +148,23 @@ class TestMockOpen(unittest.TestCase):
         expected_calls = [call('foo'), call().__enter__(), call().read(),
                           call().__exit__(None, None, None)]
         self.assertEqual(mock.mock_calls, expected_calls)
-        self.assertIs(f, handle)
+        # mock_open.return_value is no longer static, because
+        # readline support requires that it mutate state
 
+    def test_mock_open_context_manager_multiple_times(self):
+        mock = mock_open()
+        with patch('%s.open' % __name__, mock, create=True):
+            with open('foo') as f:
+                f.read()
+            with open('bar') as f:
+                f.read()
+
+        expected_calls = [
+            call('foo'), call().__enter__(), call().read(),
+            call().__exit__(None, None, None),
+            call('bar'), call().__enter__(), call().read(),
+            call().__exit__(None, None, None)]
+        self.assertEqual(mock.mock_calls, expected_calls)
 
     def test_explicit_mock(self):
         mock = MagicMock()

Misc/NEWS

@@ -66,6 +66,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21750: mock_open.read_data can now be read from each instance, as it
+  could in Python 3.3.
+
 - Issue #23247: Fix a crash in the StreamWriter.reset() of CJK codecs.
 
 - Issue #18622: unittest.mock.mock_open().reset_mock would recurse infinitely.