Celery task workers measured by coverage, but not by pytest-cov #708
Description
Summary
Running a test suite involving Celery task workers, the new coverage.py patch = subprocess feature correctly measures the workers. But with the same coverage settings, using pytest-cov does not measure the workers.
Reproducer
small_celery.py:
from celery import Celery
from celery.contrib.testing import worker
from testcontainers.redis import RedisContainer
import pytest
app = Celery("tasks", broker="redis://localhost:6379/0", backend="redis://localhost:6379/0")
@app.task
def add(x, y):
return x + y
@pytest.fixture(scope="session")
def redis_container():
with RedisContainer() as container:
yield container
@pytest.fixture
def celery_app(redis_container):
host = redis_container.get_container_host_ip()
port = redis_container.get_exposed_port(6379)
redis_url = f"redis://{host}:{port}/0"
app.conf.update(broker_url=redis_url, result_backend=redis_url)
return app
@pytest.fixture
def celery_worker(celery_app):
with worker.start_worker(
celery_app,
pool="prefork",
perform_ping_check=False,
):
yield
def test_add_task(celery_worker):
result = add.delay(4, 4)
assert result.get() == 8requirements.txt:
celery[redis]
pytest
testcontainers[redis]
pyproject.toml:
[tool.coverage.run]
patch = ["subprocess", "_exit"]Run with coverage:
% coverage run -m pytest small_celery.py; coverage combine; coverage report -m
============================= test session starts ==============================
platform darwin -- Python 3.11.13, pytest-8.4.1, pluggy-1.6.0
rootdir: /Users/nbatchelder/lab/celery/5
configfile: pyproject.toml
plugins: cov-6.2.1
collected 1 item
small_celery.py . [100%]
============================== 1 passed in 4.29s ===============================
Combined data file .coverage.MacBook-RX5KW.42996.XaDnQVWx
Combined data file .coverage.MacBook-RX5KW.42999.XxepszGx
Name Stmts Miss Cover Missing
-----------------------------------------------
small_celery.py 26 0 100%
-----------------------------------------------
TOTAL 26 0 100%
Run with pytest-cov:
% pytest --cov=. --cov-report=term-missing small_celery.py
============================= test session starts ==============================
platform darwin -- Python 3.11.13, pytest-8.4.1, pluggy-1.6.0
rootdir: /Users/nbatchelder/lab/celery/5
configfile: pyproject.toml
plugins: cov-6.2.1
collected 1 item
small_celery.py . [100%]
================================ tests coverage ================================
______________ coverage: platform darwin, python 3.11.13-final-0 _______________
Name Stmts Miss Cover Missing
-----------------------------------------------
small_celery.py 26 1 96% 10
-----------------------------------------------
TOTAL 26 1 96%
============================== 1 passed in 4.36s ===============================
Something about pytest-cov is interfering with coverage.py's new subprocess support. What can we do about that?
Versions
% uv pip freeze
Using Python 3.11.13 environment at: /Users/nbatchelder/lab/celery/.venv
amqp==5.3.1
billiard==4.2.1
celery==5.5.3
certifi==2025.8.3
charset-normalizer==3.4.3
click==8.2.1
click-didyoumean==0.3.1
click-plugins==1.1.1.2
click-repl==0.3.0
coverage==7.10.3
docker==7.1.0
idna==3.10
iniconfig==2.1.0
kombu==5.5.4
packaging==25.0
pluggy==1.6.0
prompt-toolkit==3.0.51
pygments==2.19.2
pytest==8.4.1
pytest-cov==6.2.1
python-dateutil==2.9.0.post0
python-dotenv==1.1.1
redis==5.2.1
requests==2.32.4
six==1.17.0
testcontainers==4.12.0
typing-extensions==4.14.1
tzdata==2025.2
urllib3==2.5.0
vine==5.1.0
wcwidth==0.2.13
wrapt==1.17.2