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() allows you to test if two objects are the same. The following shows the syntax of the assertIs() method:

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 datetime


class 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 unittest

from 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 Logger class in the setUp() method and assign it to the self.logger instance 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

OKCode 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 unittest


class 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

OKCode 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.