CODEWITHSUNDEEP
python
user3342397
user3342397

Reputation: 23

Python append function not working as intended

I am trying to append a list to a list using an append function. See the below code - the temp will be a temporary list which is created dynamically. I want to append temp to list1.

temp = []
list1 = []
for num in range(0,2):
    temp.clear()
    temp.append(num)
    list1.append(temp)
    print(list1)

The output intended in list1 is [[0],[1]]. However i get [[1],[1]]. Can some one explain the reason behind this. Append should append to a list with out changing the list.

Upvotes: 2

Views: 535

Answers (1)

Sam
Sam

Reputation: 1415

Since you have appended the whole temp list, then list1 contains a reference to that list. So when you modify temp with temp.clear() and temp.append(num), then list1 is also modified. If you don't want that to happen, you'll need to append a copy:

import copy
temp = []
list1 = []
for num in range(0,2):
    temp.clear()
    temp.append(num)
    list1.append(copy.copy(temp))
    print(list1)
> [[0]]
> [[0], [1]]

Another way to get the desired results is to use temp = [] instead of temp.clear(). That will make temp point to a new object in memory, leaving the one that is referenced in list1 unchanged. Both work!

Upvotes: 2

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