Python unittest

Summary: in this tutorial, you’ll learn about the unit test concept and how to use the Python unittest module to perform unit testing.

What is a unit test

A unit test is an automated test that:

  • Verifies a small piece of code called a unit. A unit can be a function or a method of a class.
  • Runs very fast.
  • Executes in an isolated manner.

The idea of unit testing is to check each small piece of your program to ensure it works properly. It’s different from integration testing which tests that different parts of the program work well together.

The goal of a unit test is to find bugs. Also, a unit test can help refactor existing code to make it more testable and robust.

Python provides you with a built-in module unittest that allows you to carry out unit testing effectively.

xUnit terminology

The unittest module follows the xUnit philosophy. It has the following major components:

  • System under test is a function, a class, a method that will be tested.
  • Test case class (unittest.TestCase): is the base class for all the test classes. In other words, all test classes are subclasses of the TestCase class in the unittest module.
  • Test fixtures are methods that execute before and after a test method executes.
  • Assertions are methods that check the behavior of the component being tested.
  • Test suite is a group of related tests executed together.
  • Test runner is a program that runs the test suite.

Python unittest example

Suppose you have Square class that has a property called length and a method area() that returns the area of the square. The Square class is in the square.py module:

class Square: def __init__(self, length) -> None: self.length = length def area(self): return self.length * self.length
Code language: Python (python)

To test the Square class, you create a new file called test_square.py file and import the unittest module like this:

import unittest
Code language: Python (python)

Since the test_square.py needs to access the Square class, you have to import it from the square.py module:

import unittest from square import Square
Code language: Python (python)

To create a test case, you define a new class called TestSquare that inherits from the TestCase class of the unittest module:

class TestSquare(unittest.TestCase): pass
Code language: Python (python)

To test the area() method, you add a method called test_area() to the TestSquare class like this:

import unittest from square import Square class TestSquare(unittest.TestCase): def test_area(self): square = Square(10) area = square.area() self.assertEqual(area, 100)
Code language: Python (python)

In the test_area() method:

  • First, create a new instance of the Square class and initialize its radius with the number 10.
  • Second, call the area() method that returns the area of the square.
  • Third, call the assertEqual() method to check if the result returned by the area() method is equal to an expected area (100).

If the area is equal to 100, the assertEqual() will pass the test. Otherwise, the assertEqual() will fail the test.

Before running the test, you need to call the main() function of the unittest module as follows:

import unittest from square import Square class TestSquare(unittest.TestCase): def test_area(self): square = Square(10) area = square.area() self.assertEqual(area, 100) if __name__ == '__main__': unittest.main()
Code language: Python (python)

To run the test, you open the terminal, navigate to the folder, and execute the following command:

python test_square.py
Code language: Python (python)

If you use Linux or macOS, you need to use the python3 command instead:

python3 test_square.py
Code language: Python (python)

It’ll output the following:

. ---------------------------------------------------------------------- Ran 1 test in 0.000s OK
Code language: plaintext (plaintext)

The output indicates that one test has passed denoted by the dot (.) If a test failed, you would see the letter F instead of the dot (.)

To get more detailed information on the test result, you pass the verbosity argument with the value 2 to the unittest.main() function:

import unittest from square import Square class TestSquare(unittest.TestCase): def test_area(self): square = Square(10) area = square.area() self.assertEqual(area, 100) if __name__ == '__main__': unittest.main(verbosity=2)
Code language: Python (python)

If you run the test again:

python test_square.py
Code language: Python (python)

you’ll get the detailed information:

test_area (__main__.TestSquare) ... ok ---------------------------------------------------------------------- Ran 1 test in 0.001s OK
Code language: plaintext (plaintext)

The output list the test case with the result ok this time instead of the dot (.)

Running tests without calling unittest.main() function

First, remove the if block that calls the unittest.main() function:

import unittest from square import Square class TestSquare(unittest.TestCase): def test_area(self): square = Square(10) area = square.area() self.assertEqual(area, 100)
Code language: Python (python)

Second, execute the following command to run the test:

python3 -m unittest
Code language: Python (python)

This command discovers all the test classes whose names start with Test* located in the test_* file and execute the test methods that start with test*. the -m option stands for the module.

In this example, the command executes the test_area() method of the TestSquare class in the test_square.py test module.

If you use macOS or Linux, you need to use the python3 command instead:

python3 -m unittest
Code language: Python (python)

It’ll return something like:

. ---------------------------------------------------------------------- Ran 1 test in 0.000s OK
Code language: Python (python)

To display more information, you can add -v option to the above command. v stands for verbosity. It’s like calling the unittest.main() with verbosity argument with value 2.

python -m unittest -v
Code language: Python (python)

Output:

test_area (test_square.TestSquare) ... ok ---------------------------------------------------------------------- Ran 1 tests in 0.000s OK
Code language: Python (python)

Testing expected exceptions

The Square constructor accepts a length parameter. The length parameter should be either an int or float. If you pass the value that is not in these types, the Square constructor should raise a TypeError exception.

To test if the Square constructor raises the TypeError exception, you use the assertRaises() method in a context manager like this:

import unittest from square import Square class TestSquare(unittest.TestCase): def test_area(self): square = Square(10) area = square.area() self.assertEqual(area, 100) def test_length_with_wrong_type(self): with self.assertRaises(TypeError): square = Square('10')
Code language: Python (python)

If you run the test again, it will fail:

python -m unittest -v
Code language: Python (python)

Output:

test_area (test_square.TestSquare) ... ok test_length_with_wrong_type (test_square.TestSquare) ... FAIL ====================================================================== FAIL: test_length_with_wrong_type (test_square.TestSquare) ---------------------------------------------------------------------- Traceback (most recent call last): File "D:\python-unit-testing\test_square.py", line 13, in test_length_with_wrong_type with self.assertRaises(TypeError): AssertionError: TypeError not raised ---------------------------------------------------------------------- Ran 2 tests in 0.001s
Code language: plaintext (plaintext)

The test_length_with_wrong_type() method expected that the Square constructor raises a TypeError exception. However, it didn’t.

To pass the test, you need to raise an exception if the type of the length property is not int or float in the Square constructor:

class Square: def __init__(self, length) -> None: if type(length) not in [int, float]: raise TypeError('Length must be an integer or float') self.length = length def area(self): return self.length * self.length
Code language: Python (python)

Now, all the tests pass:

python -m unittest -v
Code language: Python (python)

Output:

test_area (test_square.TestSquare) ... ok test_length_with_wrong_type (test_square.TestSquare) ... ok ---------------------------------------------------------------------- Ran 2 tests in 0.001s OK
Code language: Python (python)

The following example adds a test that expects a ValueError exception if the length is zero or negative:

import unittest from square import Square class TestSquare(unittest.TestCase): def test_area(self): square = Square(10) area = square.area() self.assertEqual(area, 100) def test_length_with_wrong_type(self): with self.assertRaises(TypeError): square = Square('10') def test_length_with_zero_or_negative(self): with self.assertRaises(ValueError): square = Square(0) square = Square(-1)
Code language: Python (python)

If you run the test, it’ll fail:

python -m unittest -v
Code language: Python (python)

Output:

test_area (test_square.TestSquare) ... ok test_length_with_wrong_type (test_square.TestSquare) ... ok test_length_with_zero_or_negative (test_square.TestSquare) ... FAIL ====================================================================== FAIL: test_length_with_zero_or_negative (test_square.TestSquare) ---------------------------------------------------------------------- Traceback (most recent call last): File "D:\python-unit-testing\test_square.py", line 17, in test_length_with_zero_or_negative with self.assertRaises(ValueError): AssertionError: ValueError not raised ---------------------------------------------------------------------- Ran 3 tests in 0.001s FAILED (failures=1)
Code language: Python (python)

To make the test pass, you add another check to the Square() constructor:

class Square: def __init__(self, length) -> None: if type(length) not in [int, float]: raise TypeError('Length must be an integer or float') if length < 0: raise ValueError('Length must not be negative') self.length = length def area(self): return self.length * self.length
Code language: Python (python)

Now, all three tests pass:

python -m unittest -v
Code language: Python (python)

Output:

test_area (test_square.TestSquare) ... ok test_length_with_wrong_type (test_square.TestSquare) ... ok test_length_with_zero_or_negative (test_square.TestSquare) ... ok ---------------------------------------------------------------------- Ran 3 tests in 0.001s OK
Code language: plaintext (plaintext)

Summary

  • A unit test is an automated test that verifies a small piece of code, executes fast, and executes in an isolated manner.
  • Use the unittest module to perform unit testing.
  • Create a class that inherits from the unittest.TestCase class to make a test case.
  • Use the assertEqual() method to test if two values are equal.
  • Use the assertRaises() method in a context manager to test expected exceptions.
  • Use the python -m unittest -v command to run a test.
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