FrameLocalsProxy `|=` and `update()` mishandle errors raised while merging mappings

[lpyu001]

Bug report

Bug description:

Bug description

FrameLocalsProxy does not correctly propagate exceptions raised while merging a dict subclass or another supported mapping-like object.

There are two related problems in Objects/frameobject.c:

1. framelocalsproxy_inplace_or() returns Py_NotImplemented when framelocalsproxy_merge() fails. If the merge failed because Python code raised an exception, the slot returns a normal object while an exception is still set. In a pydebug build this triggers a fatal _Py_CheckSlotResult error.
2. framelocalsproxy_update() replaces any failure from framelocalsproxy_merge() with a generic TypeError, masking the original exception.

This is a sub-issue of #146102 with gist details
A minimal reproducer is below.

import operator
import sys

class BadDict(dict):
    def keys(self):
        raise RuntimeError("keys() failed!")

def repro_update():
    locs = sys._getframe().f_locals
    print(type(locs))

    try:
        locs.update(BadDict())
    except TypeError as exc:
        print("BUG: update() masked RuntimeError as TypeError:", exc)
    except RuntimeError as exc:
        print("OK: update() propagated RuntimeError:", exc)

def repro_inplace_or():
    locs = sys._getframe().f_locals
    print(type(locs))

    try:
        operator.ior(locs, BadDict())
    except RuntimeError as exc:
        print("OK: |= propagated RuntimeError:", exc)
    except BaseException as exc:
        print("BUG: |= raised wrong exception:", type(exc).__name__, exc)
    else:
        print("BUG: |= swallowed RuntimeError")

repro_update()
repro_inplace_or()

operator.ior(locs, BadDict()) is used to exercise the same nb_inplace_or slot as locs |= BadDict().

Actual behavior

For .update(), the original RuntimeError from BadDict.keys() is replaced with a generic TypeError:

<class 'FrameLocalsProxy'>
BUG: update() masked RuntimeError as TypeError: update() argument must be dict or another FrameLocalsProxy

For |= on a pydebug build, the interpreter aborts because the slot reports success while an exception remains set:

<class 'FrameLocalsProxy'>
Fatal Python error: _Py_CheckSlotResult: Slot |= of type FrameLocalsProxy succeeded with an exception set
Python runtime state: initialized
Traceback (most recent call last):
  File "/tmp/repro_ior.py", line 6, in keys
    raise RuntimeError("keys() failed!")
RuntimeError: keys() failed!
Aborted (core dumped)

Expected behavior

Both FrameLocalsProxy.update() and FrameLocalsProxy.__ior__() should propagate the original exception raised while merging:

<class 'FrameLocalsProxy'>
OK: update() propagated RuntimeError: keys() failed!
<class 'FrameLocalsProxy'>
OK: |= propagated RuntimeError: keys() failed!

I will submit a PR to fix this

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-153418: Fix FrameLocalsProxy error propagation in update and |= #153421