input: Write a function that calculates the sum of the elements in a list of numbers. If the list is empty, the sum should be 0.
  def sum_list(arr):
      """Calculates the sum of a given list of numbers.
      
      Args:
          arr (list): List of numbers.
          
      Returns:
          int: Sum of list elements.
      """
      sum = 0
      for num in arr:
          sum += num
      return sum
  
  assert sum_list([1, 2, 3, 4, 5])==15
  assert sum_list([10, 20, 30])==60
  assert sum_list([-1, 1, -2, 2, 0])==0
  assert sum_list([])==0
output: def sum_list(arr):
      sum = 0
      for num in arr:
          sum += num
      return sum