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typing: one None-valued member makes isinstance() on a runtime_checkable Protocol O(N) per call #156413
Description
A class satisfying a @runtime_checkable Protocol with any class attribute set to None still passes isinstance(), but is orders of magnitude slower, scaling linearly with member count. Best of 5 runs of 20,000 calls, at 5/20/50/110 members: 25x/84x/316x/735x slower.
import timeit
from typing import Protocol, runtime_checkable
@runtime_checkable
class P(Protocol):
@property
def a(self) -> bool: ...
@property
def b(self) -> bool: ...
class Good:
a = b = True
class Bad:
a, b = True, None
for x in (Good(), Bad()):
print(isinstance(x, P), timeit.timeit(lambda: isinstance(x, P), number=100_000))On 3.13.13: True 0.0121 then True 0.1207. Only the timing differs.
Activity
Merge pull request #4628 from adamtheturtle/adamtheturtle/cite-cpytho…
picnixz commentedon Aug 26, 2026
adamtheturtle commentedon Aug 27, 2026
_ProtocolMeta.__instancecheck__starts withif _abc_instancecheck(cls, instance): return True._proto_hookacceptsGood, so it goes intoABCMeta's positive cache and every later call is a cache hit. It rejectsBad, which goes into the negative cache, soBadre-runs the O(N)getattr_staticloop every time.The two paths don't agree on what
= Nonemeans._proto_hooktreats anyNonein a class__dict__as "not implemented".__instancecheck__only treats it that way for callable members (attr not in cls.__non_callable_proto_members__).No, 3.12 and up. On 3.9 to 3.11 both classes take the slow path and the ratio is 1.0x. At 110 members, best of 5 runs of 20k,
Goodgoes from 26615 ns/call on 3.11.14 to 101 on 3.14.4, whileBadstays at 26645 and 52966.Goodis what changed. So this isBadmissing the 3.12 fast path rather than a slowdown.There is nothing to bisect. The asymmetry starts at the commit that added the fast path, 47753ec (Performance of typing._ProtocolMeta._get_protocol_attrs and isinstance #74690, gh-74690: typing: Simplify and optimise
_ProtocolMeta.__instancecheck__#103159, 3.12.0b1). It dropped the_is_callable_members_only(...)guard and movedsuper().__instancecheck__(instance)to the top, which is what made the ABC cache reachable for protocols with non-callable members.Badnever satisfies_proto_hook, so it was never covered by it.picnixz commentedon Aug 27, 2026
Sorry, but for me this looks like the bad commit I wanted :') I wanted to know whether it was already like that when we introduced typing.Protocol but no.
I don't think it's that bad, considering that for 100+ members you get 101ns for good classes and 50ms otherwise. However, what's annoying is that it's easy to have a property that could return
Noneor some other type and one class decides to be None while an other decides to be non-None.OTOH, I didn't use
@runtime_checkablea lot and we document it as follows:I don't have a good idea of how to tackle this correctly but if the patch is un-necessarily complex and could cause breakage, then it's also possible to leave things as is.
cc @JelleZijlstra @AlexWaygood
adamtheturtle commentedon Aug 27, 2026
Thank you for looking at this deeply @picnixz . It is a bit beyond my understanding. I asked the AI to generate a PR, and it doesn't look particularly big, but I prefer to leave that to the experts on whether it is a worthwhile trade-off.