JIT (correctness): Different UnboundLocalError for same code with JIT on or off

Bug report

Bug description:

The following code results in two different UnboundLocalError exceptions depending on whether it's run with the JIT on or off:

def f():

    if 0:
        class Class:
            pass

    if 0:
        instance = Class()

    for _ in range(5000):
        try:
            _ = instance.f()
        except Exception:
            pass
    Class.__bases__ = (object,)

f()

With JIT on, the UnboundLocalError points to 'instance', while with JIT off it points to 'Class'.

Traceback with JIT on:

Traceback (most recent call last):
  File "/home/danzin/divergences/divergence_child_6010_3_1.py", line 17, in <module>
    f()
    ~^^
  File "/home/danzin/divergences/divergence_child_6010_3_1.py", line 12, in f
    _ = instance.f()
        ^^^^^^^^
UnboundLocalError: cannot access local variable 'instance' where it is not associated with a value

Traceback with JIT off:

Traceback (most recent call last):
  File "/home/danzin/divergences/divergence_child_6010_3_1.py", line 17, in <module>
    f()
    ~^^
  File "/home/danzin/divergences/divergence_child_6010_3_1.py", line 15, in f
    Class.__bases__ = (object,)
    ^^^^^
UnboundLocalError: cannot access local variable 'Class' where it is not associated with a value

Output for PYTHON_LLTRACE=4:
divergence_01_lltrace.txt

Output for PYTHON_OPT_DEBUG=4:
divergence_01_opt_debug.txt

Please let me know whether you're able to reproduce this issue.

Found using lafleur.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

It's possible #139109 might end up fixing this inadvertently, so let's not fix this for now maybe? Or we could fix it with the knowledge the code will be ripped out soon.

Here's a simpler reproducer for what seems to be the same issue:

def f():
    for i in range(500):
        try:
            g(i)
        except Exception:
            pass

for _ in range(5000):
    f()

This one runs to completion without the JIT and raises an exception with it.

gh-140104: Set next_instr properly in the JIT during exceptions (GH-1…

We need a backport for 3.14, don't we?
Or we ignore backports since JIT is in expiremental stage?

Let's backport to 3.14 but not 3.13.

Backport failed, maybe I should open up the backport to anyone who wants to try please?

Backport failed, maybe I should open up the backport to anyone who wants to try please?

I'd like to provide this backport.

pythongh-140104: Set next_instr properly in the JIT during exceptions (…

[3.14] gh-140104: Set next_instr properly in the JIT during exceptions (

This is still broken on 3.14 sadly.

Revert "[3.14] pythongh-140104: Set next_instr properly in the JIT du…

This is still broken on 3.14 sadly.

I'm sorry. It seems that I've tested incorrectly.
It's actually still broken on 3.14.

This is still broken on 3.14 sadly.

I'm sorry. It seems that I've tested incorrectly. It's actually still broken on 3.14.

No worries. I didn't realize CI didn't trigger for JIT on the backport either!

[3.14] gh-140104: Revert "Set next_instr properly in the JIT during e…

[3.14] gh-140104: Set next_instr properly in the JIT during exceptions (

pythongh-140104: Set next_instr properly in the JIT during exceptions (…

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JIT (correctness): Different `UnboundLocalError` for same code with JIT on or off #140104

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Operating systems tested on:

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JIT (correctness): Different UnboundLocalError for same code with JIT on or off #140104

Description

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

Activity

Fidget-Spinner commented on Oct 14, 2025

devdanzin commented on Oct 15, 2025

efimov-mikhail commented on Oct 27, 2025

Fidget-Spinner commented on Oct 27, 2025

Fidget-Spinner commented on Oct 27, 2025

efimov-mikhail commented on Oct 27, 2025

Fidget-Spinner commented on Nov 1, 2025

efimov-mikhail commented on Nov 1, 2025

Fidget-Spinner commented on Nov 1, 2025

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JIT (correctness): Different `UnboundLocalError` for same code with JIT on or off #140104