input: Write a function to calculate the sum of the items in a given list and return the sum. If the given list is empty, return 0.
  
  assert sum_list([1, 2, 3, 4, 5]) == 15
  assert sum_list([-1, -2, -3, -4, -5]) == -15
  assert sum_list([0, 0, 0, 0, 0]) == 0
  assert sum_list([]) == 0
output: def sum_list(lst):
      if not lst:
          return 0
      return sum(lst)