Description

eendebakpt

opened on Nov 3, 2026

Issue body actions

Feature or enhancement

Raising exceptions in python is rather expensive:

d = {'a': 'b', 'c': 1}

def f():
    try: return d['x']
    except: pass
def g():
    if 'x' in d:
        return d['x']
%timeit f()
%timeit g()

211 ns ± 1.08 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
31.5 ns ± 0.265 ns per loop (mean ± std. dev. of 7 runs, 10,000,000 loops each)

We can improve this by using freelists (for tracebacks and exceptions), using managed dicts for exceptions and improving the performance of attribute setters.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-161242: Add a freelist for traceback objects #161243
• gh-161242: Cache strerror() messages per errno #161455
• gh-161242: Use AC vectorcall for AttributeError, ImportError, NameError #161462

Reacted by Cody Maloney

Activity

1. eendebakpt

   added

   type-featureA feature request or enhancementA feature request or enhancement

   on Nov 3, 2026
2. aisk

   added

   performancePerformance or resource usagePerformance or resource usage

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   on Nov 3, 2026
3. cmaloney commented on Nov 3, 2026

   

   cmaloney

   on Nov 3, 2026

   Contributor

   More actions

   👍 to adding a freelist.

   I'm planning to use AC Vectorcall on the core exception hierarchy including AttributeError and KeyError which should help a number of cases although have to be careful in application. Some exceptions store the args tuple and going tuple -> Vectorcall -> tuple is slower than not doing vectorcall unless take special care.

   OSError currently spends most its time re-encoding the output of strerror_r as a str which I plan to fix (just keep all the ones we've encountered as a dict of immortal strings).

   Can I use this issue for that work or would you rather a separate one?

   Reacted by Pieter Eendebak
4. cmaloney

   added 2 commits that reference this issue on Nov 6, 2026

   pythongh-161242: Cache strerror() messages per errno

   241ed72

   pythongh-161242: Cache strerror() messages per errno

   98e6d6e
5. eendebakpt

   added a commit that references this issue on Nov 6, 2026

   gh-161242: Add a freelist for traceback objects (#161243)

   3feccd1
6. cmaloney

   added a commit that references this issue on Nov 7, 2026

   gh-161242: Use AC vectorcall for AttributeError, ImportError, NameErr…

   f767423

Issue actions