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Unit Testing

unittest Module

In Python, the unittest module from the standard library provides a framework for unit testing.

It can be imported with import unittest. 

import unittest

Skip with unittest

In Python’s unittest framework, individual test methods can be skipped by using the skip, skipIf, or skipUnless decorators, as well as the TestCase .skipTest method.

The skipUnless option skips the test if the condition evaluates to False. The skipIf option skips the test if the condition evaluates to True.

In the code example, both skipUnless() and skipIf() skip the test if the operating system is not Windows.

class Tests(unittest.TestCase):


    @unittest.skipUnless(sys.platform.startswith("win"), "requires Windows")
    def test_linux_feature(self):
        print("This test should only run on windows")


    @unittest.skipIf(not sys.platform.startswith("win"), "requires Windows")
    def test_other_linux_feature(self):
        print("This test should only run on windows")

expectedFailure with unittest

In Python’s unittest framework, individual tests can be marked as an expected failure using the expectedFailure decorator. With this decorator, the test will be considered successful in test results if it fails and will be marked as a failure if it passes.

In the code example, the .expectedFailure decorator is used, and therefore, the test_something_that_fails() test will pass if assertEqual(0,1) fails (it indeed does fail and the test as a whole passes).

class Tests(unittest.TestCase):


    @unittest.expectedFailure
    def test_something_that_fails(self):
        self.assertEqual(0, 1)

Test Fixture

In Python, a test fixture is a mechanism for ensuring that proper test setup and teardown occur.

Test fixtures can be created by adding setUp() and tearDown() methods to the TestCase child class. Text fixtures created like this run before and after each individual test.

In the code example, 'This runs before each test' will be printed before each test in the class, and 'This runs after each test' will be printed after each test.

class Tests(unittest.TestCase):


    def setUp(self):
        print('This runs before each test')


    def test_equality(self):
        self.assertEqual(1, 1)


    def test_list(self):
        self.assertIn(1, [1, 2, 3])


    def tearDown(self):
        print('This runs after each test')

SyntaxError

A SyntaxError is reported by the Python interpreter when some portion of the code is incorrect. This can include misspelled keywords, missing or too many brackets or parentheses, incorrect operators, missing or too many quotation marks, or other conditions.

age = 7 + 5 = 4


# SyntaxError: can't assign to operator

TypeError

A TypeError in Python occurs when an operation is attempted on a data type that does not support it. This typically happens when functions or operators are used incorrectly with incompatible types.

result = "hello" + 5


# TypeError: can only concatenate str (not "int") to str

NameError

In Python, a NameError is an exception that is raised when referencing a variable that is not found in the current namespace. 

str = "Hello World"


print(x)


# This will throw a NameError since x is not defined.

assert Keyword

In Python, the assert keyword is used to test that a condition is met. If the condition evaluates to False, an AssertionError is raised with the specified error message.

The example code would produce the following output:

Traceback (most recent call last):
  File "/Users/me/Desktop/exceptions.py", line 2, in <module>
    assert number == 3, "number is not equal to 3!"
AssertionError: Number is not equal to 3!

number = 5
assert number == 3, 'Number is not equal to 3!'

Unit Test

In Python, a unit test validates the behavior of a single isolated component, such as a function.

Often, a function is tested for its expected return value by giving it one or two extreme inputs and one normal use case.

Test Case from unittest.TestCase

In Python, a test case is an individual test for an expected response to a given set of inputs.

In the code example, notice how test cases are created by defining a class that inherits from unittest.TestCase. Then, tests are added by defining methods that start with the word test.

class MyTests(unittest.TestCase):


    def test_example(self):
        print('This is a test method that will pass')

Test Runner with main()

In Python, a test runner is a component that collects and executes tests and then provides results to the user.

Tests can be run by calling unittest.main() in a module that has a test class derived from unittest.TestCase.

class MyTests(unittest.TestCase):


    def test_example(self):
        print('This is a test method that will pass')


unittest.main()

The example code would output:

.
----------------------------------------------------------------------
Ran 1 test in 0.000s

OK

assertEqual() from unittest

In Python’s unittest framework, the assertEqual TestCase method is used to test that two values are equal to each other.

In the code example, .assertEqual() is checking if output (which is add_five(1)) is equal to 6.

class Tests(unittest.TestCase):


    def test_add_five(self):
        output = add_five(1)
        self.assertEqual(output, 6)

assertIn() from unittest

In Python’s unittest framework, the assertIn TestCase method is used to test that a specific item is present in a container.

In the example code, .assertIn() checks if 'red' is present in colors. In this case, the test will fail since 'red' is not included in the list returned by get_rainbow_colors().

def get_rainbow_colors():
  return ['orange', 'yellow', 'green', 'blue', 'indigo', 'violet']


class Tests(unittest.TestCase):


    def test_get_rainbow_colors(self):
        colors = get_rainbow_colors()
        self.assertIn('red', colors)

assertRaises() from unittest

In Python’s unittest framework, the assertRaises TestCase method is used to test that a specific exception is raised.

The assertRaises method takes an exception type as its first argument, a function reference as its second, and an arbitrary number of arguments as the rest.

In the code example, the assertRaises() test calls the raise_index_error() function. If an IndexError exception is raised, the test passes. If any other type of exception is raised or if no exception is raised, the test will fail.

class Tests(unittest.TestCase):
    
    def test_raises_index_error(self):
        self.assertRaises(IndexError, raise_index_error)

Test Fixtures with setUpClass() and tearDownClass()

In Python’s unittest framework, test fixtures can be created by adding setUpClass() and tearDownClass() methods to the TestCase child class. Text fixtures created like this run only once - before and after all tests in the class are executed. These methods must be decorated with the classmethod decorator.

In the code example, 'This runs once before all tests' will be printed once before all the tests in the class, and 'This runs once after all tests' will be printed after all the tests.

class Tests(unittest.TestCase):
    
    @classmethod
    def setUpClass(self):
        print('This runs once before all tests')


    def test_equality(self):
        self.assertEqual(1, 1)


    def test_list(self):
        self.assertIn(1, [1, 2, 3])


    @classmethod
    def tearDownClass(self):
        print('This runs once after all tests')

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