'Convert a key into a cache file path. Basically this is the
root cache path joined with the md5sum of the key and a suffix.'

def _key_to_file(self, key, version=None):

    key = self.make_key(key, version=version)
    self.validate_key(key)
    return os.path.join(self._dir, ''.join([hashlib.md5(force_bytes(key)).hexdigest(), self.cache_suffix]))
