caplog for log messages in PyTest - Build an Automation Script

Verify log messages using caplog in pytest

Preconditions (2)

Step 1: Run the function that generates log messages

Step 2: Capture the log messages using the caplog fixture

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

✅ Expected Result: The expected log message is found in the captured logs during the test run

Automation Requirements - pytest

Assertions Needed:

Assert that the expected log message is present in caplog.text

Assert the log level of the captured message if applicable

Best Practices:

Use caplog fixture parameter in the test function

Set caplog level explicitly if needed

Avoid hardcoding log message substrings; use exact or partial matching carefully

Automated Solution

PyTest

import logging
import pytest

# Function to test

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

# Test function using caplog

def test_greet_logs_message(caplog):
    with caplog.at_level(logging.INFO):
        result = greet('Alice')
    assert result == 'Hello, Alice!'
    assert 'Greeting Alice' in caplog.text
    # Optional: check log level of the first record
    assert caplog.records[0].levelname == 'INFO'

The greet function logs an info message with the name. The test test_greet_logs_message uses the caplog fixture to capture logs.

Inside the test, caplog.at_level(logging.INFO) sets the log level to INFO to capture info messages. The function is called with 'Alice'.

Assertions check that the function returns the expected greeting string and that the log message 'Greeting Alice' appears in the captured logs. Additionally, it verifies the log level of the first captured record is INFO.

This approach ensures logs are captured and verified without printing them, making tests clean and reliable.

Common Mistakes - 3 Pitfalls

Not using caplog fixture parameter in the test function

Not setting the log level with caplog.at_level()

Asserting on printed output instead of caplog.text

Bonus Challenge

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

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