Unit Testing in Python

Let's learn about Pytest and Unittest

Before getting started

What is Unit Testing?

: A repetitive activity of verifying that individual code units within a module or application work as expected

Why Unit Testing is Necessary

• Helps you think about how to write code

• Helps determine whether implementation choices are appropriate when deciding what needs to be done

• Allows you to find bugs quickly and fix them easily

• Can improve code quality

• Simplifies integration and improves design

• Streamlines the debugging process

Python test frameworks

• Unittest

• Doctest

• Pytest

What is Unittest?

• The default test framework that comes with the Python standard library

• Not as broad in scope as other third-party test frameworks

  • Provides only the functionality needed to write unit tests suitable for most projects

• Was influenced by Java's JUnit testing framework

  • Therefore, you cannot write test functions without creating a class for testing

Unittest example

import unittest

class TestStringMethods(unittest.TestCase):

    def test_upper(self):
        self.assertEqual('foo'.upper(), 'FOO')

    def test_isupper(self):
        self.assertTrue('FOO'.isupper())
        self.assertFalse('Foo'.isupper())

    def test_split(self):
        s = 'hello world'
        self.assertEqual(s.split(), ['hello', 'world'])
        # check that s.split fails when the separator is not a string
        with self.assertRaises(TypeError):
            s.split(2)

if __name__ == '__main__':
    unittest.main()

Unittest Fixtures

: Refers to the process of setup before a test is run or clean-up after it finishes

• setUp()

  • A method called before each test method is invoked

• setUpClass()

  • A method called only once when the class starts

  • The method must have the @classmethod decorator and cls must be passed as an argument

  • ex)

    @classmethod
    def setUpClass(cls):
        ...

• tearDown()

  • A method called after each test finishes

  • Runs even if an exception occurs during the test

  • However, it is only called if the setUp() method was successful

• tearDownClass()

  • A method called only once after the class finishes

  • Similarly, the method must have the @classmethod decorator and cls must be passed as an argument

  • ex)

    @classmethod
    def tearDownClass(cls):
        ...

What is Pytest?

• Unlike Unittest, Pytest allows you to create test methods as functions that follow specific naming conventions in a module, rather than requiring a Class

Pytest Fixtures

• Provides fixtures in a unique way unlike other testing frameworks

• Makes it easy to identify which test uses which fixture

• ex)

  from handler import handler
  
  @pytest.fixture()
  def load_file():
      file = None
      try:
          file = open("sample_file.json", "r")
          content = eval(file.read())
      finally:
          if file:
              file.close()
  
      return content
  
  def test_handler(load_file):
     handler(load_file)

XFail

• Allows you to indicate that a test function is expected to fail

  • Can be used when writing tests that are supposed to fail

    • ex) Test code to prevent incorrect data types

• ex)

  from handler import handler
  
  @pytest.fixture()
  def load_wrong_format_file():
      file = None
      try:
          file = open("sample_file.txt", "r")
          content = eval(file.read())
      finally:
          if file:
              file.close()
  
      return content
  
  @pytest.mark.xfail
  def test_handler_with_wrong_format_file(self, load_wrong_format_file):
     handler(load_wrong_format_file)