gh-74690: Improve performance when testing against protocols with many attributes.

[randolf-scholz]

This adds a very simple check for detecting true negatives:

# quick true negative check for protocols with many attributes.
if not hasattr(instance, '__getattr__'):
    if not (cls.__protocol_attrs__ <= set(dir(instance))):
        return False

Running some benchmarks with pyperf against the example provided here: #74690 (comment) give the following speedups:

Benchmark	baseline	baseline_repro	test
one_int	34.9 us	not significant	12.8 us: 2.73x faster
two_frac	505 ns	501 ns: 1.01x faster	502 ns: 1.01x faster
three_float	39.2 us	not significant	10.7 us: 3.67x faster
Geometric mean	(ref)	1.05x slower	2.16x faster

one_int and three_float are negative examples, and we see a 2.7x-3.7x speedup, but no measurable slowdown for the true positive two_frac.

Are there any other benchmarks to test against?

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

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[AlexWaygood]

What about classes that define custom __dir__() methods? What about classes that define custom __getattribute__ methods? I think this approach is very risky

[randolf-scholz]

Well, is there any C-implemented alternative to dir, that returns some iterable of the attribute names? I saw inspect.getmembers_static, but it is mostly plain python and way too slow. If custom __dir__ is a worry, one could maybe use object.__dir__(instance).

dir itself is even inefficient since it sorts the names, here we really only need something that iterates over the names.

[randolf-scholz]

In regard to the custom __getattribute__ stuff, I think the current code already suffers from that, no? The docs say about getattr_static:

Note: this function may not be able to retrieve all attributes that getattr can fetch (like dynamically created attributes) and may find attributes that getattr can’t (like descriptors that raise AttributeError).

[AlexWaygood]

is there any C-implemented alternative to dir, that returns some iterable of the attribute names?

Not that I know of

[AlexWaygood]

In regard to the custom __getattribute__ stuff, I think the current code already suffers from that, no?

Ah indeed; good point

[randolf-scholz]

So really the issue here seems that

1. getattr_static is just really slow (I re-ran the test, replacing it with plain getattr and got 11x speedup)
   • getattr can be slow if it triggers some computation.
2. isinstance(x, proto) only checks statically known attributes and can give incorrect results if certain attributes are defined dynamically.
   • since Protocols are about static typing, this seems ok to do.
3. There is nothing in the data-model for classes to provide something like a __hasattr__ to communicate which attributes are set dynamically.

[AlexWaygood]

Your analysis is correct.

• getattr_static is just really slow (I re-ran the test, replacing it with plain getattr and got 11x speedup)

  • getattr can be slow if it triggers some computation.

• isinstance(x, proto) only checks statically known attributes and can give incorrect results if certain attributes are defined dynamically.

Note that we changed these two bullet points in Python 3.12 after long discussions in python/typing#1363 and #102433. Previously, isinstance() checks didn't use inspect.getattr_static, and did resolve dynamically defined attributes. But it was decided that that behaviour was bad, so we decided to accept the performance penalty in order to have more correct and less unexpected behaviour. (And through various other performance optimisations to compensate for that, isinstance() checks against runtime-checkable protocols are still overall much faster in Python 3.12 than in Python 3.11, despite the slowdown from switching to inspect.getattr_static.)

[randolf-scholz]

Wait for dir_static (#55979) and see if it still improves performance.