cProfile with list comprehension is less obvious with Python 3.12

[tim-schilling]

Bug report

I believe that the cProfiler is less helpful now in python 3.12 when attempting to point a developer to a list comprehension as the bottleneck. With python 3.11, it points out that there is a listcomp that is responsible for the bulk of the duration. In 3.12, that line is missing.

Python 3.11.4

>>> import cProfile
>>> cProfile.run('[i for i in range(50000) if i % 2 == 0]')
         4 function calls in 0.009 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.009    0.009    0.009    0.009 <string>:1(<listcomp>)
        1    0.000    0.000    0.009    0.009 <string>:1(<module>)
        1    0.000    0.000    0.009    0.009 {built-in method builtins.exec}
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}

Python 3.12.0b4

>>> cProfile.run('[i for i in range(50000) if i % 2 == 0]')
         3 function calls in 0.006 seconds

   Ordered by: standard name

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.006    0.006    0.006    0.006 <string>:1(<module>)
        1    0.000    0.000    0.006    0.006 {built-in method builtins.exec}
        1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}

Your environment

• CPython versions tested on: 3.12.0b4
• Operating system and architecture: Ubuntu 22.04.2 LTS

[AlexWaygood]

At a guess, probably a result of PEP-709? (Cc. @carljm)

[tim-schilling]

I'm sorry, I didn't catch this. https://peps.python.org/pep-0709/#tracing-profiling-will-no-longer-show-a-call-return-for-the-comprehension

Tracing/profiling will no longer show a call/return for the comprehension

Naturally, since list/dict/set comprehensions will no longer be implemented as a call to a nested function, tracing/profiling using sys.settrace or sys.setprofile will also no longer reflect that a call and return have occurred.

[AlexWaygood]

I'm sorry, I didn't catch this.

Don't be sorry — it's our job to call out backwards-incompatible changes in the docs!

@carljm, do you think the impact on tracing/profiling from PEP-709 maybe deserves a bullet point in https://docs.python.org/3.12/whatsnew/3.12.html#pep-709-comprehension-inlining?

It already has a bullet point in the PEP-709 section of "What's New in Python 3.12". Hmm, not sure how we could document this anymore thoroughly than we already have. Unless we put a note in the cProfile docs as well? Not sure where that would go, though.

[AlexWaygood]

Since this is already called out in PEP-709, and in "What's New in Python 3.12", I don't think there's any way we can further improve the docs here :/

when attempting to point a developer to a list comprehension as the bottleneck.

On the plus side, list comprehensions should also be much faster in Python 3.12 as a result of this change :-)

[tim-schilling]

Yup, yup. This makes sense. I was looking for a change in cProfile in 3.12, not with the comprehensions. Thank you @AlexWaygood!