Testing logging output with pytest

Question

I am trying to write a test, using pytest, that would check that a specific function is writing out a warning to the log when needed. For example:

In module.py:

import logging LOGGER = logging.getLogger(__name__)  def run_function():     if something_bad_happens:         LOGGER.warning('Something bad happened!') 

In test_module.py:

import logging from module import run_function  LOGGER = logging.getLogger(__name__)  def test_func():     LOGGER.info('Testing now.')     run_function()     ~ somehow get the stdout/log of run_function() ~     assert 'Something bad happened!' in output 

I have seen that you can supposedly get the log or the stdout/stderr with pytest by passing capsys or caplog as an argument to the test, and then using either capsus.readouterr() or caplog.records to access the output.

However, when I try those methods, I only see "Testing now.", and not "Something bad happened!". It seems like the logging output that is happening within the call to run_function() is not accessible from test_func()?

The same thing happens if I try a more direct method, such as sys.stdout.getvalue(). Which is confusing, because run_function() is writing to the terminal, so I would think that would be accessible from stdout...?

Basically, does anyone know how I can access that 'Something bad happened!' from within test_func()?

Answer 1

I don't know why this didn't work when I tried it before, but this solution works for me now:

In test_module.py:

import logging from module import run_function  LOGGER = logging.getLogger(__name__)  def test_func(caplog):     LOGGER.info('Testing now.')     run_function()     assert 'Something bad happened!' in caplog.text