Question

我的代码如下（我不知道为什么它没有缩进）：

def move_joker_2(deck):

#start checking every card in the deck until the position of 
#JOKER2 is found

#we start from position 0
position = 0

while deck [position] != JOKER2:
    position = position + 1

#the new position for JOKER2
if position < len (deck) - 3:
    new_position = position + 2
elif position == len (deck) - 2:
    new_position = 0
else:
    new_position = 1

#to reorganize deck, we need 2 things
# 1.) a list of the cards above JOKER2 after moving the cards
# 2.) a list of the cards below JOKER2 after moving the cards

#depending of new_position, there are 3 possible new decks

if new_position == 0:
    #no cards above, JOKER2 will become the top card
    cards_above = []
    #every other card will be below it
    cards_below = deck
    #remove JOKER2, since we moved it
    cards_below.pop(position)

elif new_position == 1:
    #the only card above JOKER2 will be the top card
    cards_above = [deck[0]]     

    #every other card up except the last one will be below it
    cards_below = deck [new_position:len(deck)-1]

else:   

    cards_above = deck[0:new_position+1]    
    #remove JOKER2, since we moved it
    cards_above.pop(position)
    cards_below = deck [new_position+1:]

#final deck
deck = cards_above + [JOKER2] + cards_below

我的代码收到一个字符串列表并最终改变它......

但为什么不改变原始列表呢？例如：

deck = [1,3,27,8,9] move_joker_2（甲板）

考虑到JOKER2是27，它应该改变列表： [1,3,8,9,27]

但每当我打电话给它时，它都没有改变......

Answer 1

deck = cards_above + [JOKER2] + cards_below不会更改deck的内容。

它创建了一个新列表，deck引用了新列表。

要更改桌面使用切片表示法的内容，例如deck[:] = cards_above + [JOKER2] + cards_below。

>>> def f1(deck):
...     deck = [1,2,3] # This does not change the `deck` passed.
                       # This just create a local variable `deck`.
...
>>> def f2(deck):
...     deck[:] = [4,5,6] # This change the `deck` passed.
...
>>> deck = [0]
>>> f1(deck)
>>> deck
[0]
>>> f2(deck)
>>> deck
[4, 5, 6]

Answer 2

作为参数传递的

deck只是一个带有参考副本的变量，因此当您指定

时

deck = ....

创建新对象并将其引用分配给deck变量。这些是c ++意义上的不引用，它们是始终引用的副本。

一个可行的解决方法是使用

deck[:] = ...

更新对象的内容，而不仅仅是参考值

为什么我的代码不会改变列表？

2 个答案: