Python assertIs()
Summary: in this tutorial, you’ll learn how to use the Python assertIs() to test if two objects are the same.
Introduction to Python assertIs() method
The assertIs()assertIs()
assertIs(first, second, msg=None)Code language: Python (python)
If the first and second reference the same object, the test will pass. Otherwise, it’ll fail.
The msg is optional. It’s displayed in the test result in case the test fails.
Technically, the assertIs() method uses the is operator:
first is second
Python assertIs() method example
First, create a Logger singleton class in the logger.py module:
from datetime import datetimeclass Logger:
_instance = None
def __new__(cls):
if cls._instance is None:
cls._instance = super(Logger, cls).__new__(cls)
return cls._instance
def log(self, message):
print(f'{datetime.now().strftime("%Y-%m-%d %H:%M:%S")} {message}')
Code language: Python (python)
The singleton is a design pattern that limits the instantiation of a class to a single instance. In other words, you’ll have the same Logger object regardless of how many times you call the Logger() constructor.
Second, create a TestLogger class that tests the Logger class:
import unittestfrom logger import Logger
class TestLogger(unittest.TestCase):
def setUp(self):
self.logger = Logger()
def test_singleton(self):
new_logger = Logger()
self.assertIs(self.logger, new_logger)
Code language: Python (python)
In the TestLogger class:
- First, create a new instance of the
Loggerclass in thesetUp()method and assign it to theself.loggerinstance variable. - Second, create a new instance of the Logger class and use the
assertIs()method to check if two instances are the same.
If you run the test:
python -m unittest -vCode language: Python (python)
you’ll get the following output:
test_singleton (test_logger.TestLogger) ... ok----------------------------------------------------------------------
Ran 1 test in 0.000s
OK
Code language: plaintext (plaintext)
The test passed.
Python assertIsNot() method
The assertIsNot() tests if the first object is not the same as the second one:
assertIsNot(first, second, msg=None)Code language: Python (python)
For example:
import unittestclass TestInteger(unittest.TestCase):
def test_integer_different_value(self):
x = 10
y = 20
self.assertIsNot(x, y)
def test_integer_same_value(self):
x = 10
y = 10
self.assertIs(x, y)
Code language: Python (python)
Run the test:
python -m unittest -vCode language: Python (python)
Output:
test_integer_different_value (test_integer.TestInteger) ... ok
test_integer_same_value (test_integer.TestInteger) ... ok----------------------------------------------------------------------
Ran 2 tests in 0.001s
OK
Code language: plaintext (plaintext)
In this example, we use the assertIsNot() method to test if two integer variables reference different objects. Since their values are different, they reference different objects.
In the second test case, we use the assertIs() method to test if two integer variables reference the same object. Because their values are the same, they reference the same object.
Summary
- Use the
assertIs()method to test if two objects are the same. - Use the
assertIsNot()method to test if two variables reference different objects.