How to write a custom `.assertFoo()` method in Python?

Question

I'm writing some test cases for my application using Python's unittest. Now I need to compare a list of objects with a list of another objects to check if the objects from the first list are what I'm expecting.

How can I write a custom .assertFoo() method? What should it do? Should it raise an exception on failure? If yes, which exception? And how to pass the error message? Should the error message be a unicode string or a bytestring?

Unfortunately, the official documentation doesn't explain how to write custom assertion methods.

If you need a real-world example for this, continue reading.

The code I'm writing is somewhat like this:

def assert_object_list(self, objs, expected):
    for index, (item, values) in enumerate(zip(objs, expected)):
        self.assertEqual(
            item.foo, values[0],
            'Item {0}: {1} != {2}'.format(index, item.foo, values[0])
        )
        self.assertEqual(
            item.bar, values[1],
            'Item {0}: {1} != {2}'.format(index, item.bar, values[1])
        )

def test_foobar(self):
    objs = [...]  # Some processing here
    expected = [
        # Expected values for ".foo" and ".bar" for each object
        (1, 'something'),
        (2, 'nothing'),
    ]
    self.assert_object_list(objs, expected)

This approach makes it extremely easy to describe the expected values of each object in a very compact way, and without needing to actually create full objects.

However... When one object fails the assertion, no further objects are compared, and this makes debugging a bit more difficult. I would like to write a custom method that would unconditionally compare all objects, and then would display all objects that failed, instead of just the first one.

This answer for the question Continuing in Python's unittest when an assertion fails should help you override the TestCase.assertEqual() correctly. And this other question Comparison of multi-line strings in Python unit test should help you in creating a custom myAssertEqual() and register it to be automatically called. — Evandro Coan
– Evandro Coan, Commented Feb 3, 2019 at 4:20

Alan Cristhian · Accepted Answer · 2013-12-16 02:22:10Z

46

I use the multiple inheritance in these cases. For example:

First. I define a class with methods that will incorporate.

import os

class CustomAssertions:
    def assertFileExists(self, path):
        if not os.path.lexists(path):
            raise AssertionError('File not exists in path "' + path + '".')

Now I define a class that inherits from unittest.TestCase and CustomAssertion

import unittest

class MyTest(unittest.TestCase, CustomAssertions):
    def test_file_exists(self):
        self.assertFileExists('any/file/path')

if __name__ == '__main__':
    unittest.main()

edited Dec 16, 2013 at 2:22

answered Apr 7, 2013 at 22:29

Alan Cristhian

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9 Comments

Josh Werts Over a year ago

Good alternative approach to subclassing - this was helpful.

Denilson Sá Maia Over a year ago

This approach is also called mixin.

Cristóbal Ganter Over a year ago

I did something similar, but I think it is very annoying that, when the test fails, the stack trace always shows the line where you raise the assertion error and not the line that calls the assertion function in your actual test. Is there a way to fix that?

shao.lo Over a year ago

@CristóbalGanter to fix the line in stack trace see stackoverflow.com/questions/49927872/…

sbc100 Over a year ago

The recommend way to signal test failures is to call self.fail() or equivalently raise self.failureException(msg). While raising and AssertionError will work for now it may not always be treated equivalently by the unittest framework: See docs.python.org/2/library/unittest.html#re-using-old-test-code

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Ned Batchelder · Accepted Answer · 2011-07-11 21:51:28Z

19

You should create your own TestCase class, derived from unittest.TestCase. Then put your custom assert method into that test case class. If your test fails, raise an AssertionError. Your message should be a string. If you want to test all objects in the list rather than stop on a failure, then collect a list of failing indexes, and after the loop over all the objects, build an assert message that summarizes your findings.

answered Jul 11, 2011 at 21:51

Ned Batchelder

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1 Comment

Waylon Over a year ago

Is there any difference in raising an AssertionError and any other Exception (with respect to custom asserts and unittest)?

parisjohn · Accepted Answer · 2017-10-11 07:36:16Z

3

Just an example to sum up with a numpy comparaison unittest

import numpy as np
class CustomTestCase(unittest.TestCase):
    def npAssertAlmostEqual(self, first, second, rtol=1e-06, atol=1e-08):
        np.testing.assert_allclose(first, second, rtol=rtol, atol=atol)


class TestVector(CustomTestCase):
    def testFunction(self):
        vx = np.random.rand(size)
        vy = np.random.rand(size)
        self.npAssertAlmostEqual(vx, vy)

edited Oct 11, 2017 at 7:36

answered Oct 10, 2017 at 16:39

parisjohn

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