Pytest's caplog fixture doesn't seem to work

Summary

Pytest’s caplog fixture is a critical part of testing. I’d love to move to loguru, but loguru doesn’t seem to work with caplog.

I’m not sure if this is user error (perhaps it’s documented somewhere? I haven’t been able to find it.), if it is some design oversight/choice, or if the problem is actually on pytest’s end.

Expected Result

Users should be able to use loguru as a drop-in replacement for the stdlib logging package and have tests that use the caplog fixture still work.

Actual Result

Drop-in replacement causes tests that use the caplog pytest fixture to fail.

Steps to Reproduce

Base test file

# test_demo.py
import pytest
import logging
logger = logging.getLogger()
logger.addHandler(logging.StreamHandler())
# from loguru import logger

def some_func(a, b):
    if a < 1:
        logger.warning("Oh no!")
    return a + b

def test_some_func_logs_warning(caplog):
    assert some_func(-1, 2) == 1
    assert "Oh no!" in caplog.text

if __name__ == "__main__":
    some_func(-1, 1)
    print("end")

Without Loguru:

$ python test_demo.py
Oh no!
end
(.venv) Previous Dir: /home/dthor
09:59:56 dthor@Thorium /home/dthor/temp/loguru
$ pytest
========================== test session starts ==========================
platform linux -- Python 3.6.7, pytest-4.3.0, py-1.8.0, pluggy-0.8.1
rootdir: /home/dthor/temp/loguru, inifile:
collected 1 item

test_demo.py .                                                    [100%]

======================= 1 passed in 0.03 seconds ========================

With Loguru:

Adjust test_demo.py by commenting out stdlib logging and uncommenting loguru:

...
# import logging
# logger = logging.getLogger()
# logger.addHandler(logging.StreamHandler())
from loguru import logger
...

$ python test_demo.py
2019-02-22 10:02:35.551 | WARNING  | __main__:some_func:9 - Oh no!
end
(.venv) Previous Dir: /home/dthor
10:02:35 dthor@Thorium /home/dthor/temp/loguru
$ pytest
========================== test session starts ==========================
platform linux -- Python 3.6.7, pytest-4.3.0, py-1.8.0, pluggy-0.8.1
rootdir: /home/dthor/temp/loguru, inifile:
collected 1 item

test_demo.py F                                                    [100%]

=============================== FAILURES ================================
______________________ test_some_func_logs_warning ______________________

caplog = <_pytest.logging.LogCaptureFixture object at 0x7f8e8b620438>

    def test_some_func_logs_warning(caplog):
        assert some_func(-1, 2) == 1
>       assert "Oh no!" in caplog.text
E       AssertionError: assert 'Oh no!' in ''
E        +  where '' = <_pytest.logging.LogCaptureFixture object at 0x7f8e8b620438>.text

test_demo.py:14: AssertionError
------------------------- Captured stderr call --------------------------
2019-02-22 10:02:37.708 | WARNING  | test_demo:some_func:9 - Oh no!
======================= 1 failed in 0.20 seconds ========================

Version information

$ python --version
Python 3.6.7
(.venv) Previous Dir: /home/dthor
10:10:03 dthor@Thorium /home/dthor/temp/loguru
$ pip list
Package                Version
---------------------- -----------
ansimarkup             1.4.0
atomicwrites           1.3.0
attrs                  18.2.0
better-exceptions-fork 0.2.1.post6
colorama               0.4.1
loguru                 0.2.5
more-itertools         6.0.0
pip                    19.0.3
pkg-resources          0.0.0
pluggy                 0.8.1
py                     1.8.0
Pygments               2.3.1
pytest                 4.3.0
setuptools             40.8.0
six                    1.12.0
(.venv) Previous Dir: /home/dthor
10:10:07 dthor@Thorium /home/dthor/temp/loguru
$ uname -a
Linux Thorium 4.4.0-17763-Microsoft #253-Microsoft Mon Dec 31 17:49:00 PST 2018 x86_64 x86_64 x86_64 GNU/Linux
(.venv) Previous Dir: /home/dthor
10:11:33 dthor@Thorium /home/dthor/temp/loguru
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.2 LTS
Release:        18.04
Codename:       bionic

[Delgan]

Please, note the documentation has been updated. The recommended way to integrate Loguru with pytest is to override the caplog fixture this way:

@pytest.fixture
def caplog(caplog):
    handler_id = logger.add(caplog.handler, format="{message}")
    yield caplog
    logger.remove(handler_id)

Using PropagateHandler could result in dead-lock if inadvertently combined with InterceptHandler see #474.

[alextremblay]

Thank you all for code samples, they’re incredibly useful. I just want to share my own, in case any of you find it useful:

# conftest.py
import logging

import pytest
from loguru import logger

@pytest.fixture
def caplog(caplog):
    """
    override and wrap the caplog fixture with one of our own
    """
    logger.remove() # remove default handler, if it exists
    logger.enable("") # enable all logs from all modules
    logging.addLevelName(5, "TRACE") # tell python logging how to interpret TRACE logs
    class PropogateHandler(logging.Handler):
        def emit(self, record):
            logging.getLogger(record.name).handle(record)
    logger.add(PropogateHandler(), format="{message} {extra}", level="TRACE") # shunt logs into the standard python logging machinery
    caplog.set_level(0) # Tell logging to handle all log levels
    yield caplog