Asserting log messages in PyTest - Build an Automation Script

Verify that a specific log message is generated during function execution

Preconditions (2)

Step 1: Run the function that should produce a log message

Step 2: Capture the log output during the function execution

Step 3: Check if the expected log message appears in the captured logs

✅ Expected Result: The expected log message is found in the captured logs, confirming the function logs correctly

Automation Requirements - pytest

Assertions Needed:

Assert that the expected log message is present in the captured logs

Best Practices:

Use pytest's caplog fixture to capture logs

Use explicit log level filtering to capture relevant logs

Keep assertions clear and focused on the expected message

Automated Solution

PyTest

import logging
import pytest

# Sample function that logs a message

def greet(name):
    logger = logging.getLogger('myapp')
    logger.info(f'Greeting {name}')
    return f'Hello, {name}!'


def test_greet_logs_message(caplog):
    with caplog.at_level(logging.INFO, logger='myapp'):
        result = greet('Alice')
    
    assert result == 'Hello, Alice!'
    assert any('Greeting Alice' in message for message in caplog.messages)

The code defines a simple function greet that logs an info message when called.

The test test_greet_logs_message uses pytest's caplog fixture to capture logs from the myapp logger at INFO level.

Inside the with caplog.at_level() block, the function is called, and logs are captured.

Assertions check that the function returns the expected greeting string and that the expected log message 'Greeting Alice' is present in the captured logs.

This approach ensures the test only captures relevant logs and verifies the logging behavior precisely.

Common Mistakes - 3 Pitfalls

{'mistake': 'Not using the caplog fixture and trying to capture logs manually', 'why_bad': "Manual log capturing is error-prone and more complex than using pytest's built-in caplog fixture", 'correct_approach': 'Use the caplog fixture provided by pytest to capture and assert log messages easily'}

{'mistake': 'Not setting the log level in caplog.at_level, causing no logs to be captured', 'why_bad': 'If the log level is not set or set too high, the expected log messages may not be captured, causing false test failures', 'correct_approach': "Use caplog.at_level with the correct log level matching the function's log calls"}

Asserting exact match on caplog.text instead of checking messages list

Bonus Challenge

Now add data-driven testing with 3 different names to verify the log messages for each

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