input: Write a python function to calculate the average of a list of numbers.
  assert avg_list([1, 2, 3, 4, 5]) == 3.0
  assert avg_list([10, 20, 30, 40, 50]) == 30.0
  assert avg_list([-5, 5, 10, -10]) == 0.0
output: def avg_list(numbers):
      return sum(numbers) / len(numbers) if numbers else 0