Python 3.9 is now generally available.

google.appengine.api.memcache.set_multi

Set multiple keys' values at once, regardless of previous contents.

mapping Dictionary of keys to values.
time Optional expiration time, either relative number of seconds from current time (up to 1 month), or an absolute Unix epoch time. By default, items never expire, though items may be evicted due to memory pressure. Float values will be rounded up to the nearest whole second.
key_prefix Prefix for to prepend to all keys.
min_compress_len Unimplemented compatibility option.
namespace a string specifying an optional namespace to use in the request.

A list of keys whose values were NOT set. On total success, this list should be empty.