input: Write a function to develop a Python program that clearly understands the concept of recursion. The program should define a recursive function to calculate the factorial of a number. If the number is 1, the function should return 1; otherwise, it should return the product of the number and the factorial of the number minus one. The program should then calculate and print the factorial of the number 5.
  assert factorial(1)==1
  assert factorial(5)==120
  assert factorial(0)==None
output: def factorial(n):
      if n == 1:
          return 1
      elif n == 0:
          return None
      else:
          return n * factorial(n - 1)