Creating an ExceptionGroup segfaults on MemoryError in debug builds

Crash report

BaseExceptionGroup_new() in Objects/exceptions.c passes the result of BaseException_new() straight to PyBaseExceptionGroupObject_CAST():

PyBaseExceptionGroupObject *self =
    PyBaseExceptionGroupObject_CAST(BaseException_new(cls, args, kwds));
if (!self) {
    goto error;
}

The cast asserts _PyBaseExceptionGroup_Check(exc), which dereferences its argument, so when BaseException_new() fails with MemoryError a debug build segfaults before reaching the NULL check.

import _testcapi

_testcapi.call_with_nomemory(0, 0, ExceptionGroup, "g", (ValueError(1),))

On main (627eabf) built with --with-pydebug, this segfaults:

#0  PyObject_TypeCheck (ob=ob@entry=0x0, type=0x555555b82380 <_PyExc_BaseExceptionGroup>) at ./Include/object.h:375
#1  0x00005555556a35c0 in PyBaseExceptionGroupObject_CAST (exc=0x0) at Objects/exceptions.c:861
#2  0x00005555556a59e3 in BaseExceptionGroup_new (type=<optimized out>, args=0x7ffff736d910, kwds=0x0) at Objects/exceptions.c:977

Release builds raise MemoryError as expected. The cast came with gh-111178, and the same code is in 3.14 and 3.15.

Checking the result of BaseException_new() before casting it fixes the crash.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

No response

Linked PRs

I can confirm the bug on windows too.

D:\workspace\cpython (main -> origin)
λ python.bat
Running Debug|x64 interpreter...
Python 3.16.0a0 (heads/main:627eabf31cf, Oct 13 2026, 12:50:19) [MSC v.1944 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import _testcapi
>>> _testcapi.call_with_nomemory(0, 0, ExceptionGroup, "g", (ValueError(1),))

gh-159294: Fix crash when creating an `ExceptionGroup` fails with `Me…

[3.15] gh-159294: Fix crash when creating an `ExceptionGroup` fails w…

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Creating an ExceptionGroup segfaults on MemoryError in debug builds #159294

Crash report

Crash report

CPython versions tested on:

Operating systems tested on:

Output from running 'python -VV' on the command line:

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Creating an ExceptionGroup segfaults on MemoryError in debug builds #159294

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Crash report

Crash report

CPython versions tested on:

Operating systems tested on:

Output from running 'python -VV' on the command line:

Linked PRs

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srinivasreddy commented on Oct 13, 2026

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