Python 3.9 is now generally available.

google.appengine.api.memcache.replace

Replaces a key's value, failing if item isn't already in memcache.

key Key to set. See docs on Client for details.
value Value to set. Any type. If complex, will be pickled.
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.
min_compress_len Ignored option for compatibility.
namespace a string specifying an optional namespace to use in the request.

True if replaced. False on RPC error or cache miss.