Testing Warnings with pytest.warns

Testing Warnings with pytest.warns

Sometimes your code emits warnings that you want to verify. pytest provides pytest.warns to capture and assert on warnings.

Basic Usage

Use pytest.warns as a context manager to check that a warning is raised:

import warnings
import pytest

def deprecated_function():
    warnings.warn("This function is deprecated", DeprecationWarning)
    return 42

def test_deprecated_warning():
    with pytest.warns(DeprecationWarning):
        deprecated_function()

Checking the Warning Message

You can verify the exact warning message:

def test_warning_message():
    with pytest.warns(DeprecationWarning, match="deprecated"):
        deprecated_function()

def test_warning_exact_message():
    with pytest.warns(DeprecationWarning, match="This function is deprecated"):
        deprecated_function()

Accessing Warning Details

Capture the warning list to inspect all warnings raised:

def test_multiple_warnings():
    with pytest.warns(DeprecationWarning) as warning_list:
        deprecated_function()
    assert len(warning_list) == 1
    assert "deprecated" in str(warning_list[0].message)

When No Warning Should Be Raised

You can verify that no warning is produced by simply not using pytest.warns:

def test_no_warning():
    # If this does NOT raise a warning, the test passes
    result = stable_function()
    assert result is not None