Issue #21750: Further fixup to be styled like other mock APIs.

Doc/library/unittest.mock.rst

@@ -1995,9 +1995,11 @@ mock_open
     :meth:`~io.IOBase.readline`, and :meth:`~io.IOBase.readlines` methods
     of the file handle to return.  Calls to those methods will take data from
     *read_data* until it is depleted.  The mock of these methods is pretty
-    simplistic.  If you need more control over the data that you are feeding to
-    the tested code you will need to customize this mock for yourself.
-    *read_data* is an empty string by default.
+    simplistic: every time the *mock* is called, the *read_data* is rewound to
+    the start.  If you need more control over the data that you are feeding to
+    the tested code you will need to customize this mock for yourself.  When that
+    is insufficient, one of the in-memory filesystem packages on `PyPI
+    <https://pypi.python.org/pypi>`_ can offer a realistic filesystem for testing.
 
 Using :func:`open` as a context manager is a great way to ensure your file handles
 are closed properly and is becoming common::

Lib/unittest/mock.py

@@ -2278,6 +2278,24 @@ 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.
     """
+    def _readlines_side_effect(*args, **kwargs):
+        if handle.readlines.return_value is not None:
+            return handle.readlines.return_value
+        return list(_state[0])
+
+    def _read_side_effect(*args, **kwargs):
+        if handle.read.return_value is not None:
+            return handle.read.return_value
+        return ''.join(_state[0])
+
+    def _readline_side_effect():
+        if handle.readline.return_value is not None:
+            while True:
+                yield handle.readline.return_value
+        for line in _state[0]:
+            yield line
+
+
     global file_spec
     if file_spec is None:
         import _io
@@ -2286,42 +2304,31 @@ def mock_open(mock=None, read_data=''):
     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
-            return list(_data)
+    handle = MagicMock(spec=file_spec)
+    handle.__enter__.return_value = handle
 
-        def _read_side_effect(*args, **kwargs):
-            if handle.read.return_value is not None:
-                return handle.read.return_value
-            return ''.join(_data)
+    _state = [_iterate_read_data(read_data), None]
 
-        def _readline_side_effect():
-            if handle.readline.return_value is not None:
-                while True:
-                    yield handle.readline.return_value
-            for line in _data:
-                yield line
+    handle.write.return_value = None
+    handle.read.return_value = None
+    handle.readline.return_value = None
+    handle.readlines.return_value = None
 
-        handle = MagicMock(spec=file_spec)
-        handle.__enter__.return_value = handle
+    handle.read.side_effect = _read_side_effect
+    _state[1] = _readline_side_effect()
+    handle.readline.side_effect = _state[1]
+    handle.readlines.side_effect = _readlines_side_effect
 
-        _data = _iterate_read_data(read_data)
+    def reset_data(*args, **kwargs):
+        _state[0] = _iterate_read_data(read_data)
+        if handle.readline.side_effect == _state[1]:
+            # Only reset the side effect if the user hasn't overridden it.
+            _state[1] = _readline_side_effect()
+            handle.readline.side_effect = _state[1]
+        return DEFAULT
 
-        handle.write.return_value = None
-        handle.read.return_value = None
-        handle.readline.return_value = None
-        handle.readlines.return_value = None
-
-        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.side_effect = make_handle
+    mock.side_effect = reset_data
+    mock.return_value = handle
     return mock
 
 

Lib/unittest/test/testmock/testmock.py

@@ -1,5 +1,6 @@
 import copy
 import sys
+import tempfile
 
 import unittest
 from unittest.test.testmock.support import is_instance
@@ -1329,8 +1330,29 @@ class MockTest(unittest.TestCase):
     def test_mock_open_reuse_issue_21750(self):
         mocked_open = mock.mock_open(read_data='data')
         f1 = mocked_open('a-name')
+        f1_data = f1.read()
         f2 = mocked_open('another-name')
-        self.assertEqual(f1.read(), f2.read())
+        f2_data = f2.read()
+        self.assertEqual(f1_data, f2_data)
+
+    def test_mock_open_write(self):
+        # Test exception in file writing write()
+        mock_namedtemp = mock.mock_open(mock.MagicMock(name='JLV'))
+        with mock.patch('tempfile.NamedTemporaryFile', mock_namedtemp):
+            mock_filehandle = mock_namedtemp.return_value
+            mock_write = mock_filehandle.write
+            mock_write.side_effect = OSError('Test 2 Error')
+            def attempt():
+                tempfile.NamedTemporaryFile().write('asd')
+            self.assertRaises(OSError, attempt)
+
+    def test_mock_open_alter_readline(self):
+        mopen = mock.mock_open(read_data='foo\nbarn')
+        mopen.return_value.readline.side_effect = lambda *args:'abc'
+        first = mopen().readline()
+        second = mopen().readline()
+        self.assertEqual('abc', first)
+        self.assertEqual('abc', second)
 
     def test_mock_parents(self):
         for Klass in Mock, MagicMock:

Lib/unittest/test/testmock/testwith.py

@@ -141,6 +141,7 @@ 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()
@@ -148,8 +149,7 @@ class TestMockOpen(unittest.TestCase):
         expected_calls = [call('foo'), call().__enter__(), call().read(),
                           call().__exit__(None, None, None)]
         self.assertEqual(mock.mock_calls, expected_calls)
-        # mock_open.return_value is no longer static, because
-        # readline support requires that it mutate state
+        self.assertIs(f, handle)
 
     def test_mock_open_context_manager_multiple_times(self):
         mock = mock_open()