Python 3.13: PyThreadState_SetAsyncExc might deliver the exception outside of the thread #139622
Description
Bug report
Bug description:
I have the following naive demo that uses PyThreadState_SetAsyncExc to implement a time-limited interruptible context manager:
import contextlib
import ctypes
import sys
import threading
import time
class Interruptible:
def __init__(self, seconds: float):
self.seconds = seconds
self.finished = False
def __enter__(self):
self._thread = threading.Thread(
target=self._interrupt_in,
args=(threading.get_ident(), self.seconds),
daemon=True,
)
self._thread.start()
return self
def __exit__(self, exc_type, exc_value, traceback):
self.finished = True
def _interrupt_in(self, tid: int, seconds: float) -> None:
time.sleep(self.seconds)
print(f"Slept for {seconds}")
if self.finished:
print("Finished, not interrupting")
return
else:
print("Not finished, interrupting")
ctypes.pythonapi.PyThreadState_SetAsyncExc(
ctypes.c_long(tid), ctypes.py_object(TimeoutError)
)
def busy_loop():
s = time.monotonic()
while time.monotonic() - s < 0.5:
pass
print(f"Python version: {sys.version}")
for i in range(100):
try:
with Interruptible(seconds=0.1):
s = time.monotonic()
while time.monotonic() - s < 0.5:
pass
# NOTE: If we replace above block with the equivalent busy_loop(), it works.
# busy_loop()
except TimeoutError:
print(f"Iteration {i + 1}: ✓ TimeoutError caught")
else:
print(f"Iteration {i + 1}: ✗ TimeoutError NOT raised")
time.sleep(0.01) # Small delay between iterationsIn Python 3.13, this might fail with something like:
❯ python /tmp/demo.py
Python version: 3.13.7 (main, Sep 29 2025, 18:42:10) [GCC 11.4.0]
Slept for 0.1
Not finished, interrupting
Iteration 1: ✓ TimeoutError caught
Slept for 0.1
Not finished, interrupting
Iteration 2: ✓ TimeoutError caught
Slept for 0.1
Not finished, interrupting
Iteration 3: ✓ TimeoutError caught
Slept for 0.1
Not finished, interrupting
Iteration 4: ✓ TimeoutError caught
Slept for 0.1
Not finished, interrupting
Traceback (most recent call last):
File "/tmp/demo.py", line 50, in <module>
while time.monotonic() - s < 0.5:
^^^^^^^^^^^^^^^^^^^^^^^^^^
TimeoutError
Meaning the TimeoutError is raised but not caught by the try/except block. A few other observations:
- If I wrap the "work" inside the context manager to a wrapper function
busy_loop(), it's NOT reproducible. - I tested Python
3.11.13,3.12.11,3.13.7, and3.14.0rc3, it is only reproducible in Python3.13.7.
CPython versions tested on:
3.13, 3.12, 3.14, 3.11
Operating systems tested on:
Linux