input: Write a function to calculate the factorial of a given integer. The function should take an integer as input and return the factorial of that integer. If the input integer is less than or equal to 1, the function should return 1. The function should be recursive, meaning it calls itself.
  
  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)==6
output: def factorial(num):
      if num <= 1:
          return 1
      else:
          return num * factorial(num - 1)