@lru_cache on functions with no arguments

[Ian Kelly]

On Thu, Aug 3, 2017 at 9:36 AM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
> On Thu, Aug 3, 2017 at 8:35 AM, Paul  Moore <p.f.moore at gmail.com> wrote:
>> On Tuesday, 1 August 2017 15:54:42 UTC+1, t... at tomforb.es  wrote:
>>> > _sentinel = object()
>>> > _val = _sentinel
>>> > def val():
>>> >     if _val is _sentinel:
>>> >         # Calculate _val
>>> >     return _val
>>> >
>>> > seems entirely sufficient for this case. Write a custom decorator if you use the idiom often enough to make it worth the effort.
>>>
>>> I did some timings with this as part of my timings above and found it to be significantly slower than lru_cache with the C extension. I had to add `nonlocal` to get `_val` to resolve, which I think kills performance a bit.
>>>
>>> I agree with the premise though, it might be worth exploring.
>>
>> It's worth pointing out that there's nothing *wrong* with using lru_cache with maxsize=None. You're going to find it hard to get a pure-Python equivalent that's faster (after all, even maintaining a single variable is still a dict lookup, which is all the cache does when LRU functionality is disabled).
>
> The single variable is only a dict lookup if it's a global. Locals and
> closures are faster.
>
> def simple_cache(function):
>     sentinel = object()
>     cached = sentinel
>
>     @functools.wraps(function)
>     def wrapper(*args, **kwargs):
>         nonlocal cached
>         if args or kwargs:
>             return function(*args, **kwargs)  # No caching with args
>         if cached is sentinel:
>             cached = function()
>         return cached
>     return wrapper
>
> *Zero* dict lookups at call-time. If that's not (marginally) faster
> than lru_cache with maxsize=None I'll eat my socks.

Reading above, I now realize you were referring to the C extension
version of lru_cache. Yes, I'm sure that's faster. I still maintain
that this has to be faster than the pure-Python version however.