def testMergeFrom(self):
    msg = map_unittest_pb2.TestMap()
    msg.map_int32_int32[12] = 34
    msg.map_int32_int32[56] = 78
    msg.map_int64_int64[22] = 33
    msg.map_int32_foreign_message[111].c = 5
    msg.map_int32_foreign_message[222].c = 10

    msg2 = map_unittest_pb2.TestMap()
    msg2.map_int32_int32[12] = 55
    msg2.map_int64_int64[88] = 99
    msg2.map_int32_foreign_message[222].c = 15
    msg2.map_int32_foreign_message[222].d = 20
    old_map_value = msg2.map_int32_foreign_message[222]

    msg2.MergeFrom(msg)
    # Compare with expected message instead of call
    # msg2.map_int32_foreign_message[222] to make sure MergeFrom does not
    # sync with repeated field and there is no duplicated keys.
    expected_msg = map_unittest_pb2.TestMap()
    expected_msg.CopyFrom(msg)
    expected_msg.map_int64_int64[88] = 99
    self.assertEqual(msg2, expected_msg)

    self.assertEqual(34, msg2.map_int32_int32[12])
    self.assertEqual(78, msg2.map_int32_int32[56])
    self.assertEqual(33, msg2.map_int64_int64[22])
    self.assertEqual(99, msg2.map_int64_int64[88])
    self.assertEqual(5, msg2.map_int32_foreign_message[111].c)
    self.assertEqual(10, msg2.map_int32_foreign_message[222].c)
    self.assertFalse(msg2.map_int32_foreign_message[222].HasField('d'))
    if api_implementation.Type() != 'cpp':
      # During the call to MergeFrom(), the C++ implementation will have
      # deallocated the underlying message, but this is very difficult to detect
      # properly. The line below is likely to cause a segmentation fault.
      # With the Python implementation, old_map_value is just 'detached' from
      # the main message. Using it will not crash of course, but since it still
      # have a reference to the parent message I'm sure we can find interesting
      # ways to cause inconsistencies.
      self.assertEqual(15, old_map_value.c)

    # Verify that there is only one entry per key, even though the MergeFrom
    # may have internally created multiple entries for a single key in the
    # list representation.
    as_dict = {}
    for key in msg2.map_int32_foreign_message:
      self.assertFalse(key in as_dict)
      as_dict[key] = msg2.map_int32_foreign_message[key].c

    self.assertEqual({111: 5, 222: 10}, as_dict)

    # Special case: test that delete of item really removes the item, even if
    # there might have physically been duplicate keys due to the previous merge.
    # This is only a special case for the C++ implementation which stores the
    # map as an array.
    del msg2.map_int32_int32[12]
    self.assertFalse(12 in msg2.map_int32_int32)

    del msg2.map_int32_foreign_message[222]
    self.assertFalse(222 in msg2.map_int32_foreign_message)
    with self.assertRaises(TypeError):
      del msg2.map_int32_foreign_message['']