Python assertTrue()

Summary: in this tutorial, you’ll learn how to use Python assertTrue() to test if an expression is True and assertFalse() method to test if an expression is False.

Introduction to the Python assertTrue() method #

The assertTrue() is a method of the TestCase class in the unittest module. The assertTrue() method tests if an expression is True:

assertTrue(expr, msg=None)Copy

If the expr is True, the test passes. Otherwise, the test fails.

The msg is optional. If you pass the msg parameter, it’ll be displayed when the test fails.

The assertTrue() method is equivalent to the following expression:

bool(expr) is TrueCopy

It is not equivalent to the following:

expr is TrueCopy

In practice, you should use the assertTrue() method to test the boolean of a variable. Also, you should not use the assertTrue() method if more specific methods are available.

For example, instead of using the assertTrue() method for the following case:

assertTrue(a == b)Copy

you should use the assertEqual() method instead:

assertEqual(a,b)Copy

Because the assertEqual() method provides a more clear error message in case the test fails.

Python assertTrue() method example #

The following example uses the assertTrue() method to test if all characters in a string are digits:

import unittest


class TestBool(unittest.TestCase):
    def test_is_digit(self):
        str = '123'
        self.assertTrue(str.isdigit())Copy

Run the test:

python -m unittest -vCopy

Output:

test_is_digit (test_bool.TestBool) ... ok

----------------------------------------------------------------------
Ran 1 test in 0.000s

OKCopy

Python assertFalse() method #

The assertFalse() method tests if an expression is False. For example:

import unittest


class TestBool(unittest.TestCase):
    def test_empty_string_is_digit(self):
        str = ''
        self.assertFalse(str.isdigit())

    def test_alpha_is_digit(self):
        str = 'a00'
        self.assertFalse(str.isdigit())Copy

Run the test:

python -m unittest -vCopy

Output:

test_alpha_is_digit (test_bool.TestBool) ... ok
test_empty_string_is_digit (test_bool.TestBool) ... ok
test_is_digit (test_bool.TestBool) ... ok

----------------------------------------------------------------------
Ran 3 tests in 0.001s

OKCopy

Summary #

• Use the assertTrue() method to test if an expression is True
• Use the assertFalse() method to test if an expression is False