Python 3.9 is now generally available.

google.appengine.api.memcache.incr

Atomically increments a key's value.

Internally, the value is a unsigned 64-bit integer. Memcache doesn't check 64-bit overflows. The value, if too large, will wrap around.

Unless an initial_value is specified, the key must already exist in the cache to be incremented. To initialize a counter, either specify initial_value or set() it to the initial value, as an ASCII decimal integer. Future get()s of the key, post-increment, will still be an ASCII decimal value.

key Key to increment. If an iterable collection, each one of the keys will be offset. See Client's docstring for details.
delta Non-negative integer value (int or long) to increment key by, defaulting to 1.
namespace a string specifying an optional namespace to use in the request.
initial_value initial value to put in the cache, if it doesn't already exist. The default value, None, will not create a cache entry if it doesn't already exist.

If key was a single value, the new long integer value, or None if key was not in the cache, could not be incremented for any other reason, or a network/RPC/server error occurred.

If key was an iterable collection, a dictionary will be returned mapping supplied keys to values, with the values having the same meaning as the singular return value of this method.

ValueError If number is negative.
TypeError If delta isn't an int or long.