pytest-parametrize-cases

What is it?

The way parametrized tests work in pytest annoys me:

import pytest


@pytest.mark.parametrize("test_input,expected", [("3+5", 8), ("2+4", 6), ("6*9", 42)])
def test_eval(test_input, expected):
    assert eval(test_input) == expecteds

Passing in argument names as a comma-separated string just looks ugly and un-Pythonic, and you have to give each case as an anonymous tuple and make sure to get the order right every time. This becomes harder the more parameters you have. It's also a bit awkward to specify the test IDs. Even when you use pytest.param there's still nothing stopping you from getting the order of the parameters wrong.

So, I wrote a simple wrapper around pytest.mark.parametrize which makes it a bit nicer to read and use. The previous example would be written:

from pytest_parametrize_cases import Case, parametrize_cases


@parametrize_cases(
    Case(test_input="3+5", expected=8),
    Case(test_input="2+4", expected=6),
    Case(test_input="6*9", expected=42)
)
def test_eval(test_input, expected):
    assert eval(test_input) == expected

It also supports optional test IDs. Using the same example as the one in the pytest docs for pytest.mark.parametrize, this is how you include them:

from datetime import datetime, timedelta

from pytest_parametrize_cases import Case, parametrize_cases

@parametrize_cases(
    Case(
        "forward",
        a=datetime(2001, 12, 12),
        b=datetime(2001, 12, 11),
        expected=timedelta(1),
    ),
    Case(
        "backward",
        a=datetime(2001, 12, 11),
        b=datetime(2001, 12, 12),
        expected=timedelta(-1),
    ),
)
def test_timedistance_v4(a, b, expected):
    diff = a - b
    assert diff == expected

That is to say, you decorate your test function with @parametrize_cases and pass in multiple instances of Case as arguments. The arguments to the Case constructor are [optionally] a positional-only string which constitutes the test ID (printed when you use the -v / --verbose flag), and then keyword arguments for each of the parameters the test function expects.

In my opinion this is much more readable and user-friendly than the default way of writing parametrized tests. Related data is kept together rather than spread out in multiple containers. Specifying the keyword arguments is mandatory, so it is always clear where each piece of data ends up in the test function (explicit is better than implicit). And it is more convenient to specify test IDs.

The parametrize_cases decorator can be stacked multiple times to give the Cartesian product of your parametrizations, in the exact same way as mark.parametrize.

Dependencies

• Python >= 3.8
• pytest

Installation

pip install pytest-parametrize-cases

Or just look in src/pytest_parametrize_cases/case.py and copypaste the two definitions to somewhere in your test suite.