Asserting warnings (pytest.warns) - Build an Automation Script

Verify that a warning is raised when calling deprecated_function

Preconditions (2)

Step 1: Import deprecated_function from the module

Step 2: Call deprecated_function inside a pytest.warns context manager for DeprecationWarning

Step 3: Capture the warning message

Step 4: Assert that the warning message contains the expected text 'deprecated'

✅ Expected Result: The test passes if a DeprecationWarning is raised and the warning message contains 'deprecated'

Automation Requirements - pytest

Assertions Needed:

Assert that DeprecationWarning is raised

Assert that warning message contains expected text

Best Practices:

Use pytest.warns context manager to catch warnings

Check warning message content for accuracy

Keep test code simple and readable

Automated Solution

PyTest

import pytest

# Example deprecated function

def deprecated_function():
    import warnings
    warnings.warn('This function is deprecated', DeprecationWarning)


def test_deprecated_function_warns():
    with pytest.warns(DeprecationWarning) as record:
        deprecated_function()
    # Assert that exactly one warning was raised
    assert len(record) == 1
    # Assert warning message contains 'deprecated'
    assert 'deprecated' in str(record[0].message)

The test imports pytest and defines a sample deprecated_function that raises a DeprecationWarning.

The test function test_deprecated_function_warns uses pytest.warns as a context manager to catch warnings of type DeprecationWarning.

Inside the context, it calls the deprecated function.

After the call, it asserts that exactly one warning was caught and that the warning message contains the word 'deprecated'.

This ensures the warning is properly raised and the message is as expected.

Common Mistakes - 3 Pitfalls

Not using pytest.warns context manager and instead using try-except

Not asserting the warning message content

Catching the wrong warning type or no warning type specified

Bonus Challenge

Now add data-driven testing to check multiple functions that raise different warnings

Show Hint