@lru_cache on functions with no arguments

[tom at tomforb.es]

As part of the Python 3 cleanup in Django there are a fair few uses of @functools.lru_cache on functions that take no arguments. A lru_cache isn't strictly needed here, but it's convenient to just cache the result. Some examples are here: https://github.com/django/django/pull/8825/files

I did some profiling and I found that using `@lru_cache(maxsize=None)` on such functions is twice as fast as a standard `@lru_cache()`, apparently because with a `maxsize` the lru_cache code requires a lock acquisition and a fair bit more state to track.

Am I right in thinking that using `maxsize=None` is best for functions that accept no arguments? Should we even be using a `lru_cache` in such situations, or write our own simple cache decorator instead?