How to use the nanopb.nanopb.generator.nanopb_generator.Field.__init__ function in nanopb

def __init__(self, struct_name, desc, field_options):
        self.fullname = struct_name + desc.name
        self.extendee_name = names_from_type_name(desc.extendee)
        Field.__init__(self, self.fullname + 'struct', desc, field_options)

        if self.rules != 'OPTIONAL':
            self.skip = True
        else:
            self.skip = False
            self.rules = 'OPTEXT'