Fix issue 207

python/google/protobuf/internal/reflection_test.py

@@ -1035,6 +1035,19 @@ class ReflectionTest(unittest.TestCase):
     self.assertEqual(222, ext2[1].bb)
     self.assertEqual(333, ext2[2].bb)
 
+  def testMergeFromBug(self):
+    message1 = unittest_pb2.TestAllTypes()
+    message2 = unittest_pb2.TestAllTypes()
+    
+    # Cause optional_nested_message to be instantiated within message1, even
+    # though it is not considered to be "present".
+    message1.optional_nested_message
+    self.assertFalse(message1.HasField('optional_nested_message'))
+    
+    # Merge into message2.  This should not instantiate the field is message2.
+    message2.MergeFrom(message1)
+    self.assertFalse(message2.HasField('optional_nested_message'))
+
   def testCopyFromSingularField(self):
     # Test copy with just a singular field.
     proto1 = unittest_pb2.TestAllTypes()

python/google/protobuf/reflection.py

@@ -943,7 +943,15 @@ def _AddMergeFromMethod(cls):
     fields = self._fields
 
     for field, value in msg._fields.iteritems():
-      if field.label == LABEL_REPEATED or field.cpp_type == CPPTYPE_MESSAGE:
+      if field.label == LABEL_REPEATED:
+        field_value = fields.get(field)
+        if field_value is None:
+          # Construct a new object to represent this field.
+          field_value = field._default_constructor(self)
+          fields[field] = field_value
+        field_value.MergeFrom(value)
+      elif field.cpp_type == CPPTYPE_MESSAGE:
+        if value._is_present_in_parent:
           field_value = fields.get(field)
           if field_value is None:
             # Construct a new object to represent this field.