Python assertIsNone()

Summary: in this tutorial, you’ll learn how to use the Python assertIsNone() method to test if an expression is None.

Introduction to the Python assertIsNone() method #

The assertIsNone() is a method of the TestCase class of the unittest module. The assertIsNone() test if an expression is None:

assertIsNone(expr, msg=None)

If the expr is None, the test passes. Otherwise, the test will fail.

The msg is optional. It’ll be displayed in the test result if the test fails.

Python assertIsNone() method examples #

Let’s take some examples of using the assertIsNone() method.

1) Using assertIsNone() with a success case #

The following example uses the assertIsNone() to test if the message variable is None:

import unittest


class TestNone(unittest.TestCase):
    def test_variable_none(self):
        message = None
        self.assertIsNone(message)

Run the test:

python -m unittest -v

Output:

test_variable_none (test_none.TestNone) ... ok

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

2) Using assertIsNone() with a failed case #

The following example uses the assertIsNone() method to test if the message variable is None:

import unittest


class TestNone(unittest.TestCase):
    def test_variable_not_none(self):
        message = 'Hello'
        self.assertIsNone(message)

Run the test:

python -m unittest -v

Output:

test_variable_not_none (test_none.TestNone) ... FAIL

======================================================================
FAIL: test_variable_not_none (test_none.TestNone)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\python-unit-testing\test_none.py", line 7, in test_variable_not_none
    self.assertIsNone(message)
AssertionError: 'Hello' is not None

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)

Since the message is 'Hello', it is not None. Therefore, the test failed.

3) Using assertIsNone() with a failed case with a message #

The following example uses the assertIsNone() to test if the message variable is None. Also, we show a message when the test fails:

import unittest


class TestNone(unittest.TestCase):
    def test_variable_not_none(self):
        message = 'Hello'
        self.assertIsNone(
            message,
            f'The message is "{message}" so it is not None.'
        )

Run the test:

python -m unittest -v

Output:

test_variable_not_none (test_none.TestNone) ... FAIL

======================================================================
FAIL: test_variable_not_none (test_none.TestNone)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\python-unit-testing\test_none.py", line 7, in test_variable_not_none
    self.assertIsNone(
AssertionError: 'Hello' is not None : The message is "Hello" so it is not None.

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)

Python assertIsNotNone() method #

The assertIsNotNone() is opposite of the assertIsNone() method. The assertIsNotNone() method tests if a variable is not None.

assertIsNotNone(expr, msg=None)

The test passes if the expr is not None or fails otherwise. For example:

import unittest


class TestNone(unittest.TestCase):
    def test_variable_is_not_none(self):
        message = 'Bye'
        self.assertIsNotNone(message)

Run the test:

python -m unittest -v

Output:

test_variable_is_not_none (test_not_none.TestNone) ... ok

----------------------------------------------------------------------
Ran 1 test in 0.001s

OK

Summary #

Use the assertIsNone() method to test if a variable is None.
use the assertIsNotNone() method to test if a variable is not None.

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