'Checks if a key exists in the cache without returning it. This is a
cheap operation that bypasses loading the actual data on the backend.
This method is optional and may not be implemented on all caches.
:param key: the key to check'

def has(self, key):

    raise NotImplementedError("%s    doesn't    have    an    efficient    implementation    of    `has`.    That    means    it    is    impossible    to    check    whether    a    key    exists    without    fully    loading    the    key's    data.    Consider    using    `self.get`    explicitly    if    you    don't    care    about    performance.")
