example_of_unittest.py

import unittest


# Will this do what I expect?
def fibonacci(n):
    """Generates the n-th Fibonacci number.
    The Fibonacci Numbers are always the sum of the previous numbers:
    fib(0) = 0
    fib(1) = 1

    fib(3) = fib(2) + fib(1)
    .. = (fib(1) + fib(0)) + fib(1)
    .. = (1 + 0) + 1
    .. = 2
    """
    if n < 0:
        raise ValueError("n must not be negative")
    if n == 0:
        return 0
    if n == 1:
        return 1

    return fibonacci(n - 1) + fibonacci(n - 2)


# The tests can be run in PyCharm via right-click on the class "Run unittests in TestFibonacci"
# Note that all values in the tests are "hard-coded" - it should be as simple as possible and no code from the tested
# function should be used in the test. Always use the function as black-box!
# This should best be in another file e.g. test.py
class TestFibonacci(unittest.TestCase):
    def test_fist_number(self):
        fib_0 = fibonacci(0)
        assert fib_0 == 0, "The first fibonacci number should be 0 but was {}".format(fib_0)

    def test_10th_number(self):
        fib_10 = fibonacci(10)
        expected = 55
        assert fib_10 == expected, "The first fibonacci number should be {expected} but was {value}".format(
            expected=expected, value=fib_10)

    # This is very important since its an edge-case for the function
    def test_negative(self):
        """Test if the function raises a ValueError for a negative argument"""
        with self.assertRaises(ValueError):
            fibonacci(-1)

    # The @unittest.skip prevents the test from running. if its removed the test will run quite long :)
    @unittest.skip("test_big_number is skipped because: This will run a long time")
    def test_big_number(self):
        fib_100 = fibonacci(100)
        expected = 354224848179261915075
        assert fib_100 == expected, "The first fibonacci number should be {expected} but was {value}".format(
            expected=expected, value=fib_100)