assertWarns & assertLogs

assertWarns & assertLogs

assertWarns verifies that a warning is issued during test execution, while assertLogs verifies that specific log messages are produced. Both are context managers for clean usage.

assertWarns

import unittest
import warnings

class TestWarnings(unittest.TestCase):
    def test_deprecated_function(self):
        with self.assertWarns(DeprecationWarning):
            deprecated_function()

    def test_user_warning(self):
        with self.assertWarns(UserWarning):
            warnings.warn("This feature is experimental")

    def test_warning_message(self):
        with self.assertWarns(UserWarning) as warned:
            risky_operation()

        self.assertEqual(len(warned.warnings), 1)
        self.assertIn("risky", str(warned.warnings[0].message))

assertLogs

class TestLogging(unittest.TestCase):
    def test_error_logged(self):
        with self.assertLogs(level='ERROR') as log:
            process_request(invalid_data)

        self.assertEqual(len(log.output), 1)
        self.assertIn("Failed to process", log.output[0])

    def test_specific_logger(self):
        with self.assertLogs('myapp.auth', level='INFO') as log:
            login_user("admin", "password123")

        self.assertIn("Login successful", log.output[0])

    def test_multiple_log_levels(self):
        with self.assertLogs(level='WARNING') as log:
            run_migration()

        self.assertTrue(len(log.output) > 0)

assertNoLogs

def test_no_error_messages(self):
    with self.assertNoLogs(level='ERROR'):
        process_valid_request()

Key Takeaway

Use assertWarns to verify deprecation or user warnings, and assertLogs to verify that your code logs the right messages at the right levels.