docs: def load_from(self, other, **kwargs):
          '''Create a :class:`Message` by merging `other` with `self`.
          Values from `other` will be copied to `self` if the value was not
          set on `self` and is set on `other`.
  
          :param other: The :class:`Message` to copy defaults from.
          :type other: :class:`Message`
          :param \*\*kwargs: Additional keyword arguments to construct
              :class:`Message` with.
          :rtype: :class:`Message`
          '''
          data = self.data()
          other_data = other.data()
          for k, v in iteritems(other_data):
              if data.get(k) is None:
                  data[k] = v
          return self.load_message(data, **kwargs)