Use 'assertIs' to check literal booleans in Python unittest

I used to use Unittest’s self.assertTrue / self.assertFalse to check both literal booleans and truthy/falsy values in Unittest. Committed the same sin while writing tests in Django.

I feel like assertTrue and assertFalse are misnomers. They don’t specifically check literal booleans, only truthy and falsy states respectively.

Consider this example:

# src.py
import unittest


class TestFoo(unittest.TestCase):
    def setUp(self):
        self.true_literal = True
        self.false_literal = False
        self.truthy = [True]
        self.falsy = []

    def is_true(self):
        self.assertTrue(self.true_literal, True)

    def is_false(self):
        self.assertFalse(self.false_literal, True)

    def is_truthy(self):
        self.assertTrue(self.truthy, True)

    def is_falsy(self):
        self.assertFalse(self.falsy, True)


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

In the above snippet, I’ve used assertTrue and assertFalse to check both literal booleans and truthy/falsy values. However, to test the literal boolean values, assertIs works better and is more explicit. Here’s how to do the above test properly:

# src.py
import unittest


class TestFoo(unittest.TestCase):
    def setUp(self):
        self.true_literal = True
        self.false_literal = False
        self.truthy = [True]
        self.falsy = []

    def is_true(self):
        self.assertIs(self.true_literal, True)

    def is_false(self):
        self.assertIs(self.false_literal, False)

    def is_truthy(self):
        self.assertTrue(self.truthy, True)

    def is_falsy(self):
        self.assertFalse(self.falsy, True)


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

Notice how I’ve used self.assertIs in the is_true and is_false methods to explicitly test out the literal boolean values. The is_truthy and is_falsy methods were kept unchanged from the previous snippet.

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