gh-74690: Further optimise typing._ProtocolMeta.__instancecheck__

[AlexWaygood]

I probably should have spotted this when doing #103034! This provides some more speedup for isinstance() checks against runtime-checkable protocols. The improvement is small, but noticeable.

Benchmarks on main:

Time taken for objects with a property: 2.53
Time taken for objects with a classvar: 2.59
Time taken for objects with an instance var: 3.18
Time taken for objects with no var: 2.38
Time taken for nominal subclass instances: 0.19
Time taken for registered subclass instances: 0.46

Benchmarks with this PR:

Time taken for objects with a property: 2.49
Time taken for objects with a classvar: 2.49
Time taken for objects with an instance var: 3.06
Time taken for objects with no var: 2.36
Time taken for nominal subclass instances: 0.18
Time taken for registered subclass instances: 0.47

Benchmark script

import time
from typing import Protocol, runtime_checkable

@runtime_checkable
class HasX(Protocol):
    x: int
    def foo(self): ...

class Foo:
    @property
    def x(self) -> int:
        return 42
    def foo(self): ...

class Bar:
    x = 42
    def foo(self): ...

class Baz:
    def __init__(self):
        self.x = 42
    def foo(self): ...

class Egg: ...

class Nominal(HasX):
    def __init__(self):
        self.x = 42

class Registered: ...

HasX.register(Registered)

num_instances = 500_000
foos = [Foo() for _ in range(num_instances)]
bars = [Bar() for _ in range(num_instances)]
bazzes = [Baz() for _ in range(num_instances)]
basket = [Egg() for _ in range(num_instances)]
nominals = [Nominal() for _ in range(num_instances)]
registereds = [Registered() for _ in range(num_instances)]


def bench(objs, title):
    start_time = time.perf_counter()
    for obj in objs:
        isinstance(obj, HasX)
    elapsed = time.perf_counter() - start_time
    print(f"{title}: {elapsed:.2f}")


bench(foos, "Time taken for objects with a property")
bench(bars, "Time taken for objects with a classvar")
bench(bazzes, "Time taken for objects with an instance var")
bench(basket, "Time taken for objects with no var")
bench(nominals, "Time taken for nominal subclass instances")
bench(registereds, "Time taken for registered subclass instances")

• Issue: Performance of typing._ProtocolMeta._get_protocol_attrs and isinstance #74690

[AlexWaygood]

Continuing to skip news for now until we've reached a decision on whether we want to consider more radical optimisations that would change behaviour (#74690 (comment)). Once I'm confident we've finished optimising this code, I'll add Whatsnew/Docs/NEWS entries for all of the optimisation PRs together :)