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()TestCase class of the unittest module. The assertIsNone()
assertIsNone(expr, msg=None)Code language: Python (python)
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)
Code language: Python (python)
Run the test:
python -m unittest -vCode language: Python (python)
Output:
test_variable_none (test_none.TestNone) ... ok
———————————————————————-
Ran 1 test in 0.000s
Code language: plaintext (plaintext)
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)
Code language: Python (python)
Run the test:
python -m unittest -vCode language: Python (python)
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)
Code language: plaintext (plaintext)
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.’
)
Code language: Python (python)
Run the test:
python -m unittest -vCode language: Python (python)
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)
Code language: plaintext (plaintext)
Python assertIsNotNone() method
The assertIsNotNone()assertIsNone() method. The assertIsNotNone()
assertIsNotNone(expr, msg=None)Code language: Python (python)
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)
Code language: Python (python)
Run the test:
python -m unittest -vCode language: Python (python)
Output:
test_variable_is_not_none (test_not_none.TestNone) ... ok
———————————————————————-
Ran 1 test in 0.001s
OK
Code language: plaintext (plaintext)
Summary
- Use the
assertIsNone()method to test if a variable isNone. - use the
assertIsNotNone()method to test if a variable is notNone.