全局范围的变量不返回更新的版本

Question

我的变量value_knap正在while循环范围内更新。但是，当我在while循环之外访问它的值时，它只是回到其原始值。我以为我把它变成了函数范围内的全局变量。我尝试阅读其他博客文章，例如Python Global not updating和python read thed ocs python教程。我只是不知道我在做什么错。任何帮助将不胜感激。

def get_optimal_value (capacity, values, weights):

  #values for each item v/w by index
  val_item =[]

  #the total value of the knapsack
  value_knap = 0

  #while the bag is not full - when capacity is greater 0
  #calculate the value of each item
  #find the value of each weight
  for i in range (len (weights)):
      ratio = values[i] / weights[i] 
      val_item.append([ratio, weights[i]])

  #sort from largest to smallest    
  val_item.sort(reverse = True)
  #print(val_item)

  #while there are items available go through the list of loot items >:)
  while len(val_item) > 0:

      #if the weight is more than the capacity of the loot bag
      #return a fraction of the value of the item

      if capacity<val_item[0][1]:
          capacity = capacity / val_item[0][1]
          #print(val_item)
          value_knap = capacity * values[0] 
          val_item.pop(0) 

      #if the the bag can hold the the entire weight of the largest value item "just put it the bag...."   
      elif capacity >= val_item[0][1]:
          #update the weight of the bag
          capacity = capacity - val_item[0][1]

          #value * weight
          #print(val_item[0][0]*val_item[0][1])
          value_knap = value_knap + (val_item[0][0]*val_item[0][1])
          print(value_knap)
          val_item.pop(0)
  print(value_knap)




  return value_knap 





if __name__== "__main__":
  print (get_optimal_value (50,[60, 100, 120],[20, 50, 30]))
  #print(get_optimal_value(1000,[500],[30]))
  #print(get_optimal_value(10,[500],[30]))
  #print(500/30)