为什么我的代码没有更改列表？

Question

My code is the following (i don't know why it isn't indented):

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

My code receives a list of strings and mutates it in the end...

But why doesn't it change the original list? For example:

deck = [1, 3, 27, 8, 9] move_joker_2(deck)

It should change the list, considering that JOKER2 is 27, to: [1, 3, 8, 9, 27]

But whenever I call deck it hasn't changed...

Answer 1

deck = cards_above + [JOKER2] + cards_below does not change the content of the deck .

It make a new list and deck reference that new list.

To change the content of the desk use slice notation like 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

deck passed as an argument is just a variable with the copy of the reference so when you assign

deck = ....

you create the new object and assign its reference to the deck variable. These are not references in the c++ sense, these are always copies of references.

One opssible walkaround would be to use

deck[:] = ...

which updates the content of the object, not just the reference value