Python 3.13: PyThreadState_SetAsyncExc might deliver the exception outside of the thread

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 iterations

In 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, and 3.14.0rc3, it is only reproducible in Python 3.13.7.

CPython versions tested on:

3.13, 3.12, 3.14, 3.11

Operating systems tested on:

Linux

One plausible related change in 3.13 is #115194

@gpshead

@gpshead FYI, this isn't related to asyncio, though name of this function could be confusing.

Bisecting shows that the regressing PR was #106141 and was fixed by #122934, so this appears to be an issue with the eval breaker rather than PyThreadState_SetAsyncExc.

I'm not sure that PR the bisect shows fixing it was intended to fix this? that might be a side effect of the particular reproducer code vs that change.

Yeah, definitely not an intentional fix. I think the problem is that the eval breaker is being checked somewhere it shouldn't (or not being checked somewhere it should), and changing when while statements jump apparently fixed that.

I think I'm seeing this in Python 3.14.2 as well, in the test suite of https://github.com/glenfant/stopit. The error is visible in https://bugs.debian.org/1123317. This test looks something like this:

   >>> import threading
   >>> import time
   >>> from stopit import async_raise

   >>> def variable_duration_func(duration):
   ...     t0 = time.time()
   ...     while True:
   ...         dummy = 0
   ...         if time.time() - t0 > duration:
   ...             break
   >>> exc_traces = []
   >>> def variable_duration_func_handling_exc(duration, exc_traces):
   ...     try:
   ...         t0 = time.time()
   ...         while True:
   ...             dummy = 0
   ...             if time.time() - t0 > duration:
   ...                 break
   ...     except Exception as exc:
   ...         exc_traces.append(exc)

   >>> five_seconds_threads = threading.Thread(
   ...     target=variable_duration_func_handling_exc, args=(5.0, exc_traces))
   >>> start_time = time.time()
   >>> five_seconds_threads.start()
   >>> thread_ident = five_seconds_threads.ident
   >>> five_seconds_threads.is_alive()
   True

   >>> async_raise(thread_ident, LookupError)

async_raise calls PyThreadState_SetAsyncExc via ctypes. The same approach suggested in this issue (replacing the body of the try in variable_duration_func_handling_exc with a call to variable_duration_func(duration)) seems to work around it.

Is it possible that something changed between 3.14.0rc3 and 3.14.2 that would have introduced this bug?

test: the real-runaway bounded_join test in the engine's shape — Pyth…

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Python 3.13: PyThreadState_SetAsyncExc might deliver the exception outside of the thread #139622

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Python 3.13: PyThreadState_SetAsyncExc might deliver the exception outside of the thread #139622

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

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yilei commented on Oct 5, 2025

Eclips4 commented on Oct 5, 2025

ZeroIntensity commented on Oct 6, 2025

gpshead commented on Oct 6, 2025

ZeroIntensity commented on Oct 6, 2025

cjwatson commented on Jan 27, 2026

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