input: Write a function to find the maximum number from a list of integers. If the list is empty, return 0.
  def max_from_list(numbers):
      if not numbers:
          return 0
      max_num = numbers[0]
      for num in numbers:
          if num > max_num:
              max_num = num
      return max_num
  
  assert max_from_list([3, 4, 5]) == 5
  assert max_from_list([-1, -9, 0]) == 0
  assert max_from_list([10, 20, 30, 40]) == 40
  assert max_from_list([5]) == 5
  assert max_from_list([]) == 0
output: def max_from_list(numbers):
      if not numbers:
          return 0
      max_num = numbers[0]
      for num in numbers:
          if num > max_num:
              max_num = num
      return max_num