input: Write a function to calculate the average of a list of numbers. If the list is empty, return 0.
  def avg_list(lst):
   sum = 0
   count = 0
   for i in lst:
    sum += i
    count += 1
   if count == 0:
    return 0
   avg = sum / count
   return avg
  
  assert avg_list([1, 2, 3, 4])==2.5
  assert avg_list([5, 10, 15])==10.0
  assert avg_list([7])==7.0
  assert avg_list([])==0
output: def avg_list(lst):
      if not lst:
          return 0
      return sum(lst) / len(lst)