How to enable the programs under test logging during pytest run

[schorschie]

Hello!

Description

I have a script, which is performing some stuff and logs the results into a log file using the logging package. One of my tests checks, whether the expected output in the log file is present.

If I run the test function with python, then it works.
If I run the test function with pytest, then the log file is not created and the corresponding test fails.

I have searched the documentation and forums for a switch to enable logging, but I couldn't find one. I hope someone here can help.

Here is a code snippet to reproduces the issue:

# log_example.py

import logging
import unittest
from collections import deque

global LOG_PATH
LOG_PATH = 'my_log.log'

logging.basicConfig(filename=LOG_PATH, level=logging.DEBUG)
logger = logging.getLogger('my_logger')

def my_function():
    logger.info('Hello World')
    
class test_my_example(unittest.TestCase):
    
    def test_my_function(self):
        my_function()
        with open(LOG_PATH, 'r') as f:
            lines = deque(f, 1)
            self.assertEqual(lines[0], 'INFO:my_logger:Hello World\n')

if __name__ == '__main__':
    unittest.main()

To reproduce the error run the following command:

python.exe .\log_example.py; del my_log.log; pytest.exe .\log_example.py

The first command will print OK, whilst the second one will print FAILED (because it did not log anything)

Question

Which command line argument of pytest.exe enables the logging function of the program under test?

[schorschie]

Ok, I solved it myself.
The trick is to call the function, which creates the log entry within the assertLogs context manager of the unittest framework.

Here is a working code example, which even looks cleaner 😄

# log_example.py
 
import logging
import unittest

def my_function():
    logging.basicConfig(filename='my_log.log', level=logging.DEBUG)
    logger = logging.getLogger('my_logger')

    logger.info('Hello World')
    
class test_my_example(unittest.TestCase):
    
    def test_my_function(self):
        with self.assertLogs('my_logger', level=logging.DEBUG) as cm:
            my_function()
            self.assertEqual(cm.output, ['INFO:my_logger:Hello World'])

if __name__ == '__main__':
    unittest.main()

Run file:

python.exe .\log_example.py; del my_log.log; pytest.exe .\log_example.py

You can call the function like above. The fun thing is, that the pytest call will succeed, without even creating a log file 😆