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Comparing Django Q Objects in Python 3 with pytest

Date: 2017-05-30 22:00
tags:topic:testing, language:python, topic:django
summary:An updated simple assertion helper for comparing instances of Django's Q objects using pytest in Python 3.


In a previous post I wrote about comparing Django's Q object instances. The original code was Python 2 with unittest and was due for an update.

The previous issue with comparing Django's Q objects remains the same:

Django's Q object does not implement __cmp__ and neither does Node which it extends (Node is in the django.utils.tree module).

Unfortunately, that means that comparison of Q objects that are equal fails.

A simple Python 3 solution

The following is a Python 3.6 assertion helper for use with pytest that uses the original strategy of comparing the string versions of the Q objects.

from django.db.models import Q

def assert_q_equal(left, right):
    Test two Q objects for equality. Does is not match commutative.

        left (Q)
        right (Q)

        AssertionError: When -
            * `left` or `right` are not an instance of `Q`
            * `left` and `right` are not considered equal.
    assert isinstance(left, Q), f'{left.__class__} is not subclass of Q'
    assert isinstance(right, Q), f'{right.__class__} is not subclass of Q'
    assert str(left) == str(right), f'Q{left} != Q{right}'

This time the helper is just a function rather than a mixin for unittest.TestCase.

isinstance is used for comparison so that any instance of a class derived from Q can also be matched. The assertions have secondary expressions in the form of f-strings to give helpful output without raising a custom assertion.

When two Q instances do not match, pytest shows the following output:

______________________ test_neq_multi_not_commutative ______________________ in test_neq_multi_not_commutative
    assert_q_equal(q_a, q_b) in assert_q_equal
    assert str(left) == str(right), f'Q{left} != Q{right}'
E   AssertionError: Q(AND: ('speed', 12), ('direction', 'north')) != Q(AND: ('direction', 'north'), ('speed', 12))
E   assert "(AND: ('spee...n', 'north'))" == "(AND: ('direc...'speed', 12))"
E     - (AND: ('speed', 12), ('direction', 'north'))
E     + (AND: ('direction', 'north'), ('speed', 12))
==================== 1 failed, 7 passed in 0.07 seconds ====================

The important thing is to adjust your assertion helpers to best fit the needs of your test suite and team.

Final testing related note

Thanks to Adam's feedback on my initial post, I improved the assertions to use isinstance and secondary expressions to provide helpful output.

Tests for assert_q_equal and its original code are in this gist.

Happy testing!