Python 3.13 Error Walking Stack w/ PYTHONASYNCIODEBUG=1

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#730 opened on Mar 5, 2026 2 comments 1 reaction 0 assignees View on GitHub

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Assessment

Difficulty: 4/5
Estimated time: 3-5 days
Newbie friendliness: 45/100

Issue type: Bug
Clarity: Mostly clear
Activity status: Quiet
Tech stack: python
Domain: backend, networking

Research direction

Start by running the provided Docker reproduction from Dockerfile.latest with repro.py under Python 3.13 and PYTHONASYNCIODEBUG=1, then compare it with the environment where the variable is unset. Trace the stack-walking failure involving the async generator and C frames; done means the reproduction exits successfully with debug mode enabled.

Written by the indexing model from the issue text.

Description

There appears to be a bug in uvloop for python 3.13 walking the stack when PYTHONASYNCIODEBUG=1 and there are c frames in the stack.

I suspect this is tied to changes in 3.13 with how python handles frames

To Repro:
Python 3.13
uvloop 0.22.1 (also confirmed it happens

run: docker build -f Dockerfile.latest -t crash-test-latest . 2>&1 | tail -3 && echo "---" && docker run --rm crash-test-latest 2>&1; echo "exit: $?"
exits w/ 139

If you go into the dockerfile and set ENV PYTHONASYNCIODEBUG= (instead of 1), exits w/ 0

Dockerfile:

FROM python:3.13-slim
RUN pip install uvloop==0.22.1
ENV PYTHONASYNCIODEBUG=1
COPY repro.py /repro.py
CMD ["python3", "/repro.py"]

Repro.py

import asyncio
import sys
import os

print(f"Python: {sys.version}")
print(f"Platform: {sys.platform}")
print(f"PYTHONASYNCIODEBUG: {os.environ.get('PYTHONASYNCIODEBUG', '<not set>')}")

import uvloop
print(f"uvloop: {uvloop.__version__}")

async def gen():
    yield 1
    yield 2

async def main():
    x = gen()
    await x.__anext__()
    # x goes out of scope on function exit

uvloop.install()
asyncio.run(main())
print("NO CRASH")

Dominant language: Cython
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