On 3 January 2016 at 13:00, u8y7541 The Awesome Person surya.subbarao1@gmail.com wrote:
The wrapper functions themselves, though, exist in a one:one correspondence with the functions they're applied to - when you apply functools.lru_cache to a function, the transient decorator produced by the decorator factory only lasts as long as the execution of the function definition, but the wrapper function lasts for as long as the wrapped function does, and gets invoked every time that function is called (and if a function is performance critical enough for the results to be worth caching, then it's likely performance critical enough to be thinking about micro-optimisations). (Nick Coghlan)
Yes, that is what I was thinking of. Just like Quake's fast inverse square root. Even though it is a micro-optimization, it greatly affects how fast the game runs.
For Python, much bigger performance pay-offs are available without changing the code by adopting tools like PyPy, Cython and Numba. Worrying about micro-optimisations like this usually only makes sense if a profiler has identified the code as a hotspot for your particular workload (and sometimes not even then).
But, as I explained, the function will _not_ be redefined and trashed every frame; it will be created one time. (Andrew Barnert)
Hmm... Nick says different...
This all suggests that if your application is severely memory constrained (e.g. it's running on an embedded interpreter like MicroPython), then it *might* make sense to incur the extra complexity of using classes with a custom __call__ method to define wrapper functions, over just using a nested function. (Nick Coghlan)
The memory difference is only per function defined using the wrapper, not per call. The second speed difference I described (how long the CALL_FUNCTION opcode takes) is per call, and there native functions are the clear winner (followed by bound methods, and custom callable objects a relatively distant third).
The other thing to keep in mind is that the examples I showed were focused specifically on measuring the differences in overhead, so the function bodies don't actually do anything, and the class instances didn't contain any state of their own. Adding even a single instance/closure variable is likely to swamp the differences in memory consumption between a native function and a class instance.
Cheers, Nick.