def __repr__(self):
        return "<Requirement(package={package}, version='{version}')>".format(
            package=self.package, version=self.version)
def set_submodule(self, submodule: 'Submodule') -> None:
        """Set this instance's submodule. It must be called before
        the first write operation begins"""
        self._smref = weakref.ref(submodule)