Testing concurrent.futures.TimeoutError and logging in a threaded function using Pytest

I’ve come across a testing challenge in my Python project and I’m hoping to get some insights from the community. I have a utility module containing a function threaded_execute which utilizes the concurrent.futures module to execute a function in a separate thread. If a concurrent.futures.TimeoutError occurs, it logs a warning and retries the function. I’m using pytest for testing and I want to specifically test the logging of the TimeoutError without installing any additional packages.

Here’s a simplified version of my code:

import logging
from concurrent import futures

logger = logging.getLogger(__name__)

THREAD_TIMEOUT: float = 60  # For simplicity, just hardcoding the value here

def threaded_execute(func, *args, timeout=None, **kwargs):
    timeout = timeout or THREAD_TIMEOUT
    while True:
        with futures.ThreadPoolExecutor(max_workers=1) as executor:
            future = executor.submit(func, *args, **kwargs)
            try:
                return future.result(timeout=timeout)
            except futures.TimeoutError as exc:
                logger.warning(exc)
                continue

>Solution :

I’d write a mock executee that takes its sweet time on the first invocation, and none on the next:

import logging

import time
from concurrent import futures

logger = logging.getLogger(__name__)


def threaded_execute(func, *args, timeout: float, **kwargs):
    while True:
        with futures.ThreadPoolExecutor(max_workers=1) as executor:
            future = executor.submit(func, *args, **kwargs)
            try:
                return future.result(timeout=timeout)
            except futures.TimeoutError as exc:
                logger.warning("Timed out", exc_info=True)
                continue


def test_threaded_execute(caplog):
    i = 0

    def fun():
        nonlocal i
        i += 1
        if i == 1:  # first time around, time out
            time.sleep(1)
        return f"OK {i}"

    assert threaded_execute(fun, timeout=0.5) == "OK 2"
    assert caplog.records[0].message == "Timed out"