How to test loguru logger with unittest?

I’ve got a project which is not using pytest. How can I successfully assert log statements made by loguru using unittests?

For example, I’ve got:

# test_loguru.py

import contextlib
import io
from unittest import TestCase

from loguru import logger


class LoggerTest(TestCase):
    def test_logger_stderr(self):
        f = io.StringIO()
        with contextlib.redirect_stderr(f):
            logger.info("loguru log right here")

        self.assertEqual("loguru log right here\n", f.getvalue())

Running the test reveals I’m unsuccessful in capturing the loguru output:

$ python -m unittest test_loguru.py
2022-03-14 15:01:40.597 | INFO     | test_loguru:test_logger_stderr:12 - loguru log right here
F
======================================================================
FAIL: test_logger_stderr (test_loguru.LoggerTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/fredrik/code/repos/logger/test_loguru.py", line 14, in test_logger_stderr
    self.assertEqual("loguru log right here\n", f.getvalue())
AssertionError: 'loguru log right here\n' != ''
- loguru log right here


----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)

[Delgan]

Good point, I had forgotten that the format had to be adjusted as well!

I added the snippet to the documentation: Replacing assertLogs() method from unittest library.

Thanks for bringing this to my attention. 👍

[fredrikaverpil]

Very nice, this is a lot cleaner! 😃

I’m also thinking I’ll add in an arguments level and format (with some sane default), so I can tell individual tests what I’m actually probing for. Most of the time I don’t want to bother about the datetime string for example.

from contextlib import contextmanager

@contextmanager
def capture_logs(
    level="INFO", format="{time:YYYY-MM-DD HH:mm:ss.SSS} | {level: <8} | {name}:{function}:{line} - {message}"
):
    """Capture loguru-based logs."""
    output = []
    handler_id = logger.add(output.append, level=level, format=format)
    yield output
    logger.remove(handler_id)

Would be great to have this in the docs, yes! 😄