code: def _get_updated_values(before_values, after_values):
      
      assert before_values.keys() == after_values.keys()
      return dict([(k, [before_values[k], after_values[k]])
                   for k in before_values.keys()
                   if before_values[k] != after_values[k]])
docs: Get updated values from 2 dicts of values
  
      Args:
          before_values (dict): values before update
          after_values (dict): values after update
  
      Returns:
          dict: a diff dict with key is field key, value is tuple of
                (before_value, after_value)