Testing Django forms with pytest parameterization

Working on a largish Django project, I have to test a lot of web forms. My basic approach is to put data into the form and check if it validates. I started out using separate tests for valid and invalid input data, also thinking about for loops to handle different data sets. But you don’t want to do that.

Pytest’s Parametrizing offers a really neat and concise solution to this problem. Consider this simple example:

from django import forms

import pytest


class ExampleForm(forms.Form):
    name = forms.CharField(required=True)
    age = forms.IntegerField(min_value=18)


@pytest.mark.parametrize(
    'name, age, validity',
    [('Hugo', 18, True),
     ('Egon', 17, False),
     ('Balder', None, False),
     ('', 18, False),
     (None, 18, False),
     ])
def test_example_form(name, age, validity):
    form = ExampleForm(data={
        'name': name,
        'age': age,
    })

    assert form.is_valid() is validity

Here, three values are parameterized: the input for the two form fields and, given this data, whether or not the form should validate or not.

When working in a TDD-style, I start with one test case, code the form logic, then add the next test case by adding just one line of parameters. And repeat.

If you think there is a better way to test Django forms, please drop me a line on the Twitter!

Update 10/10/2017: pytest parameter matrices