def _add_message_field(self, field_name, value, params):
if '<STR_LIT:.>' not in field_name:<EOL><INDENT>params[field_name] = value<EOL>return<EOL><DEDENT>root, remaining = field_name.split('<STR_LIT:.>', <NUM_LIT:1>)<EOL>sub_params = params.setdefault(root, {})<EOL>self._add_message_field(remaining, value, sub_params)<EOL>
Converts a . delimitied field name to a message field in parameters.

        This adds the field to the params dict, broken out so that message
        parameters appear as sub-dicts within the outer param.

        For example:
          {'a.b.c': ['foo']}
        becomes:
          {'a': {'b': {'c': ['foo']}}}

        Args:
          field_name: A string containing the '.' delimitied name to be converted
            into a dictionary.
          value: The value to be set.
          params: The dictionary holding all the parameters, where the value is
            eventually set.