删除python引用，改为复制

Question

所以我在这个函数中有一个非常奇怪的错误，我认为它一定与python引用有关，而不是与值分配有关。如果有人能看到是什么导致mainList的价值消失，我将不胜感激。-http://pastebin.com/6sZCwAk8

inputs:
ships -> [0]
hitListLength = 1
output of first print statement [[0]]
output of second print statement [[]]

我的理论是，因为我从countingList中弹出，它从mainList中删除了值，不知道如何绕过它？

def addPlacement(ships,hitListLength,start = None, mainList = None,countingList = None):
    if start == None:
        start=0
    if mainList == None:
        mainList = []
    if countingList == None:
        countingList = []
    #pop ship from array for use on this recusion level
    ship = ships.pop()
    #loop through each hit
    for x in range(start,hitListLength):
        #add this rotation to the counting list
        countingList.append(x)
        #if we don't need to go any deeper add the counting array as an element of the main list
        if len(ships) == 0:
            mainList.append(countingList)
            print "MainList: " + str(mainList)
        else:
            #otherwise recure deeper updating mainlist
            mainList += addPlacement(ships,hitListLength,(start+1),mainList,countinglist)
        #remove this loops countingList contribution so next loop can take its place
        countingList.pop()
    #return the mainlist
    ships.append(ship)
    print "MainList: " + str(mainList)
    return mainList

Answer 1

您执行

mainList.append(countingList)

然后

countingList.pop()

它只从countingList中删除元素，在mainList 中留下[]

由于append附加了对countingList的引用，并且您从countingList中弹出了唯一的元素，因此此更改也反映在mainList和中

如果您想在mainList 中存储countingList的副本，请使用copy