Time for action – writing a unit test

We will write tests for a simple factorial function. The tests will check for the so called happy path and for abnormal conditions.

  1. We start by writing the factorial function
    def factorial(n):
  if n == 0:
    return 1

  if n < 0:
    raise ValueError, "Unexpected negative value"

  return np.arange(1, n+1).cumprod()

    The code is using the arange and cumprod functions we have already seen to create arrays and calculate the cumulative product, but we added a few checks for boundary conditions.

  2. Now we will write the unit test. Let's write a class that will contain the unit tests. It extends the TestCase class from the unittest module which is part of standard Python. We test for calling the factorial function with:
    • a positive number, the happy path
    • boundary condition 0
    • negative numbers, which should result in an error
      class FactorialTest(unittest.TestCase):
  def test_factorial(self):
    #Test for the factorial of 3 that should pass.
    self.assertEqual(6, factorial(3)[-1])
    np.testing.assert_equal(np.array([1, 2, 6]), factorial(3))

  def test_zero(self):
    #Test for the factorial of 0 that should pass.
    self.assertEqual(1, factorial(0))

  def test_negative(self):
    #Test for the factorial of negative numbers that should fail.
    # It should throw a ValueError, but we expect IndexError
    self.assertRaises(IndexError, factorial(-10))

      We rigged one of the tests to fail as you can see in the following output:

      $ python unit_test.py 
.E.
======================================================================
ERROR: test_negative (__main__.FactorialTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "unit_test.py", line 26, in test_negative
self.assertRaises(IndexError, factorial(-10))
  File "unit_test.py", line 9, in factorial
raiseValueError, "Unexpected negative value"
ValueError: Unexpected negative value

----------------------------------------------------------------------
Ran 3 tests in 0.003s

FAILED (errors=1)

What just happened?

We made some happy path tests for factorial function code. We let the boundary condition test fail on purpose (see unit_test.py):

import numpy as np
import unittest


def factorial(n):
  if n == 0:
    return 1

  if n < 0:
    raise ValueError, "Unexpected negative value"

  return np.arange(1, n+1).cumprod()


class FactorialTest(unittest.TestCase):
  def test_factorial(self):
    #Test for the factorial of 3 that should pass.
    self.assertEqual(6, factorial(3)[-1])
    np.testing.assert_equal(np.array([1, 2, 6]), factorial(3))

  def test_zero(self):
    #Test for the factorial of 0 that should pass.
    self.assertEqual(1, factorial(0))

  def test_negative(self):
    #Test for the factorial of negative numbers that should fail.
    # It should throw a ValueError, but we expect IndexError
    self.assertRaises(IndexError, factorial(-10))


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