input: Write a function to calculate the sum of all elements in a given list. If the list is empty, return 0.
  assert sum_list([1, 2, 3, 4, 5]) == 15
  assert sum_list([10, 20, 30]) == 60
  assert sum_list([]) == 0
  assert sum_list([7]) == 7
  assert sum_list([-1, 1]) == 0
output: def sum_list(lst):
      return sum(lst) if lst else 0