In https://docs.python.org/3/faq/programming.html#why-are-default-values-shared-between-objects there is an example showing memoization using `def expensive(arg1, arg2, _cache={}):`.  We should change the signature in that example to make `_cache` a keyword-only parameter and possibly adjust the comment to note that that's been done to prevent accidental calls with three positional arguments.



Note to existing contributors: if this wouldn't be within your first couple of patches, please leave this for a new contributor to find.  Thanks!