List of keys to look up. Keys may be strings or
tuples of (hash_value, string). Google App Engine
does the sharding and hashing automatically, though, so the hash
value is ignored. To memcache, keys are just series of bytes,
and not in any particular encoding.
Prefix to prepend to all keys when talking to the server;
not included in the returned dictionary.
a string specifying an optional namespace to use in
If True, request and store CAS ids on the client.
A dictionary of the keys and values that were present in memcache.
Even if the key_prefix was specified, that key_prefix won't be on
the keys in the returned dictionary.