def append(self, value):
    """Appends one element by copying the message."""
    new_element = self._message_descriptor._concrete_class()
    new_element._SetListener(self._message_listener)
    new_element.CopyFrom(value)
    self._values.append(new_element)
    if not self._message_listener.dirty:
      self._message_listener.Modified()
Appends one element by copying the message.
deps/protobuf/python/google/protobuf/internal/containers.py