Calculate factorials

Write a function factorial that given a positive integer n calculates the factorial. Remember that the factorial of n is defined as the product of all positive integers from 1 (that is, the natural numbers) to n. For example:

\[5! = 5\ x\ 4\ x\ 3\ x\ 2\ x\ 1\]

Write pytests to test your implementation. Remember that \(0! = 1\) and \(1! = 1\).

Insist that the students test their programs by adding a line
telling them to do it.