TIL #094 – pytest.raises: parameter match

pytest.raises: parameter match

Today I attended a pytest tutorial at PyCon DE 2024 and very early in the tutorial I learned about the parameter match that you can use with pytest.raises.

The parameter match accepts a regex pattern that is tested against the error message and your test only passes if the error message matches the regex.

The initial example given was this:

def parse_pos_number(s: str) -> int:
    n = int(s)
    if n < 0:
        raise ValueError("No negativity allowed!")
    return n

This function raises a ValueError if you pass it a negative value but it will also raise a (different) ValueError if you pass it a string that doesn't look like an integer:

parse_pos_number("-1")  # ValueError: No negativity allowed!
parse_pos_number("a")   # ValueError: invalid literal for int() with base 10: 'a'

So, when writing a test to make sure that parse_pos_number fails when giving it a negative number, we should also ensure that the ValueError is “the correct one”. The test would look like this:

import pytest

def test_with_negative_number():
    with pytest.raises(ValueError, match="No negativity allowed!"):
        parse_pos_number("-1")

The trainer also put forward that this parameter match can be quite useful if you have a (custom) error that is raised in multiple situations. Adding match to your tests will also test your error messages and it acts as a “documentation” test.

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References

• pytest docs, https://docs.pytest.org/en/8.0.x/reference/reference.html#pytest.raises [last accessed 22-04-2024];
• “pytest tips and tricks for a better testsuite” PyCon DE 2024 tutorial, https://github.com/The-Compiler/pytest-tips-and-tricks [last accessed 22-04-2024];