What is the logical error for this Python program to generate all possible unique ways to represent n=3 as sum of positive integers?

Question

Python program to generate all possible unique ways to represent n=3 as sum of positive integers:

def fun():
    res=[]
    a=[]
    def backtracking(n):
        if(n==0):
            res.append(a)
            print(res)
            return
        if(n<0):
            return
        for i in range(1,n+1):
            a.append(i)
            backtracking(n-i)
            a.pop()
    backtracking(3)
    return res

print(fun())  

Expecting res = [[1,1,1][1,2][2,1][3]] instead getting [ [] [] [] [] ]

Answer 1

You are appending the list a directly to the res, you should be appending a copy of the list a instead. List is passed by reference, so in the end, your res has 4 references to the same list which is empty. To get a copy of the list you have different options - list.copy() , copy.copy() method, or just slicing list[:]

def fun():
    res=[]
    a=[]
    def backtracking(n):
        if(n==0):
            res.append(a.copy())  # updated
            # print(res)
            return
        if(n<0):
            return
        for i in range(1,n+1):
            a.append(i)
            backtracking(n-i)
            a.pop()
    backtracking(3)
    return res

print(fun())

Output:

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