Asserting log messages in PyTest

Introduction

We check log messages to make sure our program reports important events correctly. This helps us find problems and understand what happened during a test.

When you want to confirm that a warning or error message is shown during a test.

When your code logs important steps and you want to verify those steps happened.

When debugging and you want to ensure specific log messages appear for certain actions.

When testing that sensitive information is not logged by mistake.

When you want to check that your logging setup works as expected.

Syntax

PyTest

with caplog.at_level(logging.LEVEL):
    # code that triggers logging
    assert 'expected message' in caplog.text

caplog is a pytest fixture that captures log messages during tests.

You can set the log level with caplog.at_level() to capture messages of that level or higher.

Examples

This test checks that a warning message is logged.

PyTest

def test_warning_log(caplog):
    import logging
    with caplog.at_level(logging.WARNING):
        logging.warning('This is a warning')
    assert 'This is a warning' in caplog.text

This test asserts the error message appears in the captured logs.

PyTest

def test_error_log(caplog):
    import logging
    with caplog.at_level(logging.ERROR):
        logging.error('Error happened')
    assert any('Error happened' in msg for msg in caplog.messages)

Sample Program

This test runs a function that logs messages at different levels. It then checks that all these messages appear in the captured logs.

PyTest

import logging
import pytest

def function_that_logs():
    logging.info('Starting process')
    logging.warning('A warning occurred')
    logging.error('An error occurred')

def test_logs(caplog):
    with caplog.at_level(logging.INFO):
        function_that_logs()
    assert 'Starting process' in caplog.text
    assert 'A warning occurred' in caplog.text
    assert 'An error occurred' in caplog.text

if __name__ == '__main__':
    pytest.main([__file__])

Important Notes

Always import logging to use log levels and log messages.

Use caplog.text to get all captured logs as one string, or caplog.records for detailed log records.

Set the log level carefully to capture the messages you want.

Summary

Use caplog fixture to capture logs in pytest tests.

Check log messages with assertions on caplog.text or caplog.messages.

Set log level with caplog.at_level() to control which messages are captured.