Python passing a list to a class function

Question

Python passes the lists as a reference to a function. is the behavior any different when calling class function using self pointer? Why the finalResultList is not printing all the values and only empty brackets. Is it something to do with recursion?

Code:

class Solution:
    def getAllPermutations(self,nums,index,resultList,finalResultList):
        if(index == len(nums)):
            finalResultList.append(resultList)
            print(finalResultList)
            return
        for i in range(len(nums)):
            if nums[i] not in resultList:
                resultList.append(nums[i])
                self.getAllPermutations(nums,index+1,resultList,finalResultList)
                if(len(resultList) != 0):
                    resultList.pop(-1)
        

    def permute(self, nums: List[int]) -> List[List[int]]:
        resultList = []
        finalResultList = []
        self.getAllPermutations(nums,0,resultList,finalResultList)
        print(finalResultList )

Output :

[[1, 2, 3]]
[[1, 3, 2], [1, 3, 2]]
[[2, 1, 3], [2, 1, 3], [2, 1, 3]]
[[2, 3, 1], [2, 3, 1], [2, 3, 1], [2, 3, 1]]
[[3, 1, 2], [3, 1, 2], [3, 1, 2], [3, 1, 2], [3, 1, 2]]
[[3, 2, 1], [3, 2, 1], [3, 2, 1], [3, 2, 1], [3, 2, 1], [3, 2, 1]]
[[], [], [], [], [], []]

self.resultList ?

sahasrara62
– sahasrara62

2022-04-10 11:45:02 +00:00
Commented Apr 10, 2022 at 11:45 — sahasrara62
– sahasrara62, Commented Apr 10, 2022 at 11:45

MatsLindh · Accepted Answer · 2022-04-10 11:38:16Z

No, the behaviour is the same. When you append the resultList to your finalResultList, you're appending the same list each time, and that list still gets changed while your code is running. As you mentioned yourself, you have the same pointer/reference to the resultList all the way through the code, including what you add to your finalResultList.

finalResultList.append(resultList)

This part just adds the same list (resultList) x number of times to the finalResultList, and you keep changing this list.

If you want to add a copy of the list that doesn't get changed, create a new list when adding it to your finalResultList:

finalResultList.append(list(resultList))

.. or use the copy module to create a copy:

finalResultList.append(copy.copy(resultList))

.. or if the resultList list had been a deep list (i.e. having internal references to other lists as well):

finalResultList.append(copy.deepcopy(resultList))

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