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