返回的列表与 function 内部的列表不同

Question

Here is my code:

def solve(lists,sums,num_list_,num,a):
    if len(num_list_)==num:
        if add(lists,num_list_)==sums:
            print(num_list_)
            return a+[num_list_]
        return a
    num_list=copy.deepcopy(num_list_)
    for x in range(10):
        num_list[num]=x
        a=solve(lists,sums,num_list,num+1,a)
    return a

def add(lists,unkown_num_list):
    answer=0
    for x in lists:
        st=''
        for y in x:
            if isinstance(y,int):
                y=str(unkown_num_list[y])
            st+=y
        st=int(st)
        answer+=st
    return answer

unkown_num_list=[]
difference=[]
a=solve([[0], [1, 0], [2, 1, 0], [3, 2, 1, 0]],2000,[0, 0, 0, 0],0,[])

It printed:

[0, 0, 0, 2]
[0, 0, 5, 1]
[5, 6, 4, 1]
[5, 6, 9, 0]

But for variable a , it is this:

[[0, 0, 0, 9], [0, 0, 5, 9], [5, 6, 4, 9], [5, 6, 9, 9]]

Why are these two different?

Is it because I used recursion? But other times, they don't have this kind of error.

I am using python3.6.1 and macOS High Sierra.

I am a beginner in Python, please point out any incorrect code.

Answer 1

Although you have a deepcopy in your solve function, in the for loop you still mutate the same num_list list after having added it to the result. So in cases where the for loop has more than one iteration, you'll get unexpected results.

One solution is to move the deepcopy inside the for loop:

def solve(lists,sums,num_list_,num,a):
    if len(num_list_)==num:
        if add(lists,num_list_)==sums:
            print(num_list_)
            return a+[num_list_]
        return a
    for x in range(10):
        num_list=copy.deepcopy(num_list_) # <----------
        num_list[num]=x
        a=solve(lists,sums,num_list,num+1,a)
    return a