How to Test Logging in pytest: Simple Guide with Examples

To test logging in pytest, use the built-in caplog fixture which captures log messages during test execution. You can assert on caplog.records or caplog.text to verify that expected logs were emitted.

Syntax

The caplog fixture in pytest captures log messages during a test. You can access:

• caplog.records: a list of log records with attributes like levelname and message.
• caplog.text: a string with all captured log messages.
• You can also set the log level with caplog.set_level() to capture logs of specific severity.

python

def test_logging_example(caplog):
    import logging
    logger = logging.getLogger('mylogger')
    caplog.set_level(logging.INFO)
    logger.info('Info message')
    assert 'Info message' in caplog.text

Example

This example shows how to test that a logger emits an error message. It sets the log level to ERROR and asserts the message is captured.

python

import logging

def function_that_logs():
    logger = logging.getLogger('myapp')
    logger.error('An error occurred')

def test_error_logging(caplog):
    caplog.set_level(logging.ERROR, logger='myapp')
    function_that_logs()
    assert any(record.levelname == 'ERROR' and 'error occurred' in record.message for record in caplog.records)

Output

============================= test session starts =============================
collected 1 item

test_logging.py .                                                          [100%]

============================== 1 passed in 0.01s ==============================

Common Pitfalls

Common mistakes when testing logging in pytest include:

• Not setting the log level with caplog.set_level(), so logs below WARNING are not captured by default.
• Checking caplog.text without considering log level or logger name filters.
• Assuming logs are captured automatically without using the caplog fixture.

python

import logging

def test_wrong_logging(caplog):
    logger = logging.getLogger('test')
    logger.info('Info message')
    # This will fail because default level is WARNING
    assert 'Info message' not in caplog.text


def test_right_logging(caplog):
    logger = logging.getLogger('test')
    caplog.set_level(logging.INFO)
    logger.info('Info message')
    assert 'Info message' in caplog.text

Quick Reference

• caplog.set_level(level, logger=None): Set minimum log level to capture.
• caplog.records: List of log records to inspect details.
• caplog.text: All captured logs as a string.
• Use any() with caplog.records to check for specific log messages and levels.

Key Takeaways

Use pytest's built-in caplog fixture to capture and test log messages.

Always set the appropriate log level with caplog.set_level() to capture desired logs.

Assert on caplog.records for detailed log info or caplog.text for raw log output.

Remember logs below WARNING are not captured by default without setting level.

Check logger names if your code uses multiple loggers to avoid false negatives.