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gh-139424: microoptimize _collections_abc._check_methods #139401

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gh-139424: microoptimize _collections_abc._check_methods #139401

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8 changes: 4 additions & 4 deletions Lib/_collections_abc.py
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Original file line number Diff line number Diff line change
Expand Up @@ -106,11 +106,11 @@ async def _ag(): yield
### ONE-TRICK PONIES ###

def _check_methods(C, *methods):
mro = C.__mro__
mro_dicts = [B.__dict__ for B in C.__mro__]
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Maybe we should use a tuple instead of a list to reduce memory usage, since this collection isn’t modified:

Suggested change
mro_dicts = [B.__dict__ for B in C.__mro__]
mro_dicts = tuple(B.__dict__ for B in C.__mro__)

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would it be reasonable to cache this tuple for re-use?

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But the caching here would need to be more sophisticated, since when the class is updated, the result must also change. Standard caching only takes the input into account, so in this case the input would be cached, but the output should actually differ depending on the class state.

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can a class mro change after the fact?

I know the contents of dict could change but if the cache is just holding references to each class' dict itself, then any modifications of any dict should already be reflected in the cache

If the mro can change this becomes invalid

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to be clear I'm not suggesting to cache the result of this function, only the mro dict tuple

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Ah, got it. I think caching the MRO dict tuple should be safe.

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Here you also need to decide what exactly we want to optimize, since a tuple gives about ~10% better memory efficiency, while a list is about ~10% faster when iterating in loops.

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interesting, I didn't know the lists are faster to iterate thru, even with the length check at each iteration. TIL, thanks

In that case I'd opt for the list for top speed in the subclass check but I'm not sure how that fits into the best practices here or what the people want

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It's not worth debating tiny differences like this until we've verified that the change itself is worth pursing.

for method in methods:
for B in mro:
if method in B.__dict__:
if B.__dict__[method] is None:
for base_dict in mro_dicts:
if method in base_dict:
if base_dict[method] is None:
return NotImplemented
break
Comment on lines +111 to 115
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Suggested change
for base_dict in mro_dicts:
if method in base_dict:
if base_dict[method] is None:
return NotImplemented
break
for base_dict in mro_dicts:
method_impl = base_dict.get(method, sentinel)
if method_impl is None:
return NotImplemented
if method_impl is not sentinel:
break

This avoids calculating hash and looking for key in dict twice. sentinel object can be created at the top of the method.

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That looks like a clear win, good eye! Might make sense to do the check optimistically, ie: consider the sentinel more likely than the None

if method_impl is sentinel:
    continue
if method_impl is None:
    return NotImplemented
break

else:
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