input: Write a function to calculate the factorial of a given integer. The factorial of a number n is the product of all positive integers less than or equal to n. 
  
  def factorial(num):
      if num <= 1:
          return 1
      else:
          return num * factorial(num - 1)
  
  assert factorial(7) == 5040
  assert factorial(0) == 1
  assert factorial(1) == 1
  assert factorial(5) == 120
  assert factorial(-3) == 1
output: def factorial(num):
      if num <= 1:
          return 1
      else:
          return num * factorial(num - 1)