input: Write a function to calculate the sum of the items in a given list. If the list is empty, return 0.
  def sum_list(input_list):
    sum = 0
    for i in input_list:
      sum += i
    return sum
  assert sum_list([1, 2, 3])==6
  assert sum_list([-1, -2, -3])==-6
  assert sum_list([10, 20, 30, 40])==100
  assert sum_list([])==0
  assert sum_list([1])==1
output: def sum_list(input_list):
    sum = 0
    for i in input_list:
      sum += i
    return sum