Optimize Model._meta._property_names

[Adam Johnson]

Continuing my project to optimize the system checks, I found some optimizations for Options._meta._property_names, which I found to take ~4% of the total runtime for checks.

Most of this function’s runtime was being spent running inspect.getattr_static(). This is not surprising as it ​jumps through many hoops in order to avoid triggering attribute access.

I added use of getattr_static() back in #28269 / ed244199c72f5bbf33ab4547e06e69873d7271d0 to fix a bug with instance descriptors. But I think it’s overly cautious, and we can assume that accessing the __dict__ of the model class will work fine.

Two optimizations make the function run in negligible time:

1. Changing the function to use __dict__ directly
2. Caching on a per-class basis. This requires using a weak-reference to classes, as we shouldn’t mutate base classes in the MRO, some of which can be non-model subclasses, like Model itself for the pk property, object, or any mixins.

Before optimization stats:

106 calls to _property_names took 26ms, or ~4% of the total runtime of system checks.

After optimization:

The same calls take 1ms, or ~0.2% of the total runtime. (The real runtime may be <1ms, but shows as 1 due to rounding up by cProfile.)