def get(self, link, package_name):
        # type: (Link, Optional[str]) -> Link
        """Returns a link to a cached item if it exists, otherwise returns the
        passed link.
        """
        raise NotImplementedError()
Returns a link to a cached item if it exists, otherwise returns the
        passed link.
        
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_internal/cache.py