def FromJsonString(self, value):
    """Converts string to FieldMask according to proto3 JSON spec."""
    if not isinstance(value, str):
      raise ValueError('FieldMask JSON value not a string: {!r}'.format(value))
    self.Clear()
    if value:
      for path in value.split(','):
        self.paths.append(_CamelCaseToSnakeCase(path))
Converts string to FieldMask according to proto3 JSON spec.
deps/protobuf/python/google/protobuf/internal/well_known_types.py