bpo-32933: Implement dunder iter method on mock_open

Lib/unittest/mock.py

@@ -2364,6 +2364,12 @@ def _readline_side_effect():
         while True:
             yield type(read_data)()
 
+    def _iter_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:
@@ -2387,6 +2393,7 @@ def _readline_side_effect():
     _state[1] = _readline_side_effect()
     handle.readline.side_effect = _state[1]
     handle.readlines.side_effect = _readlines_side_effect
+    handle.__iter__.side_effect = _iter_side_effect
 
     def reset_data(*args, **kwargs):
         _state[0] = _iterate_read_data(read_data)

Lib/unittest/test/testmock/testmock.py

@@ -1450,6 +1450,14 @@ def test_mock_open_reuse_issue_21750(self):
         f2_data = f2.read()
         self.assertEqual(f1_data, f2_data)
 
+    def test_mock_open_dunder_iter_issue_32933(self):
+        mocked_open = mock.mock_open(read_data='Remarkable Bird\nThe Norwegian Blue\nBeautiful Plumage')
+        f1 = mocked_open('a-name')
+        lines = [line for line in f1]
+        self.assertEqual(lines[0], 'Remarkable Bird\n')
+        self.assertEqual(lines[1], 'The Norwegian Blue\n')
+        self.assertEqual(lines[2], 'Beautiful Plumage')
+
     def test_mock_open_write(self):
         # Test exception in file writing write()
         mock_namedtemp = mock.mock_open(mock.MagicMock(name='JLV'))

Lib/unittest/test/testmock/testwith.py

@@ -205,6 +205,24 @@ def test_readline_data(self):
             result = h.readline()
         self.assertEqual(result, 'foo')
 
+    def test_dunder_iter_data(self):
+        # Check that dunder_iter will return all the lines from the fake file
+        # Added to test Issue 32933
+        mock = mock_open(read_data='foo\nbar\nbaz\n')
+        with patch('%s.open' % __name__, mock, create=True):
+            h = open('bar')
+            lines = [l for l in h]
+        self.assertEqual(lines[0], 'foo\n')
+        self.assertEqual(lines[1], 'bar\n')
+        self.assertEqual(lines[2], 'baz\n')
+
+        # Check that we properly emulate a file that doesn't end in a newline
+        mock = mock_open(read_data='foo')
+        with patch('%s.open' % __name__, mock, create=True):
+            h = open('bar')
+            result = h.readline()
+        self.assertEqual(result, 'foo')
+
 
     def test_readlines_data(self):
         # Test that emulating a file that ends in a newline character works