Start by writing the factorial function:

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()

The code uses the arange() and cumprod() functions to create arrays and calculate the cumulative product, but we added a few checks for boundary conditions.

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. Test for calling the factorial function with the following three attributes: