Python - List of List of strings

Question

So, basically I have a Queue (List of List of strings). Now, suppose I want to append 'okay' to the list at index 1, why is it getting appended for all the lists inside Queue? How do I append a value to a specific list only inside Queue?

Queue = []
temp = []
Queue.append(temp) # 12 - 8
Queue.append(temp)
Queue.append(temp)
Queue[1].append('okay')
print(Queue)

[['okay'], ['okay'], ['okay']]

You are appending multiple copies of exactly the same list `temp` to `Queue`. You need to append *new* lists instead. — John Coleman, Jul 21 '20 at 12:20
They're all the same list, so ... they're the same. append(temp.copy()) or in some cases deepcopy() — Kenny Ostrom, Jul 21 '20 at 12:20

score 1 · Answer 1 · answered Jul 21 '20 at 12:21

Your Queue is a list of the exact same list

Queue.append(temp) does not create a new list and add it to Queue. Therefore when you change one element of Queue, you are actually changing them all.

Queue.append([]) should be used to populate the queue with separate, empty lists.

score 1 · Answer 2 · answered Jul 21 '20 at 12:22

Because the three items in Queue are not three different empty lists: they're all temp. Thus temp, Queue[0], Queue[1] and Queue[2] are all the same object and modifying one of them has the result of modifying them all.

Try the following code instead:

Queue = []
Queue.append([])
Queue.append([])
Queue.append([])
Queue[1].append('okay')
print(Queue)

score 0 · Answer 3 · answered Jul 21 '20 at 12:29

0

replace temp = [] with list()

Queue = [ ]
Queue.append(list()) # 12 - 8
Queue.append(list())
Queue.append(list())
Queue[1].append('okay')
print(Queue)
output: [[], ['okay'], []]

answered Jul 21 '20 at 12:29

klv0000

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Python - List of List of strings

3 Answers3