why am i getting None in my list in Recursion in python

my slack is getting o/p as slack=[[],[],[]] output is not appending in it why?
candidates = [10,1,2,7,6,1,5], target = 8 Output: [ [1,1,6], [1,2,5], [1,7], [2,6] ]

type here
class Solution:
    def __init__(self):
        self.slack =[]

    def combinationSum2(self, candidates: List[int], target: int) -> List[List[int]]:
        output =[]
        i =0
        def rec(i,target,output,candidates):
            if i>=len(candidates):
                if target == 0: 
                    self.slack.append(output) 
                return self.slack
            
            if candidates[i]<=target:
                output.append(candidates[i])
                rec(i+1,target-candidates[i],output,candidates)
                output.pop()
            rec(i+1,target,output,candidates)
        
        rec(i,target,output,candidates)
        return self.slack

output needs to be appended inside the slack

>Solution :

Try this

from typing import List

class Solution:
    def __init__(self):
        self.slack = []

    def combinationSum2(self, candidates: List[int], target: int) -> List[List[int]]:
        output = []
        i = 0
        candidates.sort()  # Sort the candidates list to handle duplicates

        def rec(i, target, output, candidates):
            if i >= len(candidates):
                if target == 0:
                    self.slack.append(output.copy())  # Append a copy of 'output' to 'self.slack'
                return self.slack

            if candidates[i] <= target:
                output.append(candidates[i])
                rec(i + 1, target - candidates[i], output, candidates)

                # Skip duplicates
                while i + 1 < len(candidates) and candidates[i] == candidates[i + 1]:
                    i += 1

                output.pop()  # Remove the last element as we backtrack
            rec(i + 1, target, output, candidates)

        rec(i, target, output, candidates)
        return self.slack

Testing

candidates = [10, 1, 2, 7, 6, 1, 5]
target = 8

solution = Solution()
result = solution.combinationSum2(candidates, target)
print(result) # [[1, 1, 6], [1, 2, 5], [1, 7], [2, 6]]
assert result ==  [[1,1,6], [1,2,5], [1,7], [2,6]]