Python function changes list content even if it copied

August 30, 2023

Yo, I have a problem with function which is made for converting list of objects to list of str to be witten to json file. I copy that original list and send it to function, but the function still rewrites the original one. Here is the code

columns = [
            [Stone("O"),Stone("O")],
            [],
            [],
            [],
            [],
            [Stone("X"),Stone("X"),Stone("X"),Stone("X"),Stone("X")],
            [],
            [Stone("X"),Stone("X"),Stone("X")],
            [],
            [],
            [],
            [Stone("O"),Stone("O"),Stone("O"),Stone("O"),Stone("O")],
            [Stone("X"),Stone("X"),Stone("X"),Stone("X"),Stone("X")],
            [],
            [],
            [],
            [Stone("O"),Stone("O"),Stone("O")],
            [],
            [Stone("O"),Stone("O"),Stone("O"),Stone("O"),Stone("O"),],
            [],
            [],
            [],
            [],
            [Stone("X"),Stone("X")]

    ]

def save(array):
        i = 0
        j = 0
        print("Saving....")
        while i < len(array):
            while j < len(array[i]):
                if array[i][j].Char == "O":
                    array[i][j] = "O"
                elif array[i][j].Char == "X":
                    array[i][j] = "X"
                else:
                    continue
                j+=1
            i+=1
            j=0
        print("Saved!")
        array = json.dumps(array)
        jsonFile = open("save.json", "w")
        jsonFile.write(array)
        jsonFile.close()

arr = columns.copy()
save(arr)
print(columns[0][0])

the output should throw memory address of columns[0][0], because there is an object, but it throws 0, because the object is replaced in function "save"

I tried to comment line with "save(arr)" and the result is what i need but it doesnt save data into json obviously. Also tried to check ids of lists and arr = columns.copy() has different id than columns, but it still rewrites it

>Solution :

You’ve have a list of lists. You’ve copied the top-level, but not the inner lists. Consider the below simple example where we perform both a shallow and a deep copy on a list of lists.

>>> a = [[1,2,3], [4,5,6]]
>>> b = a.copy()
>>> b[0][2] = 'foo'
>>> a
[[1, 2, 'foo'], [4, 5, 6]]
>>> import copy
>>> a = [[1,2,3], [4,5,6]]
>>> b = copy.deepcopy(a)
>>> a
[[1, 2, 3], [4, 5, 6]]
>>> b[0][2] = 'foo'
>>> a
[[1, 2, 3], [4, 5, 6]]
>>> b
[[1, 2, 'foo'], [4, 5, 6]]

Further, checking the IDs of the objects.

>>> a = [[1,2,3], [4,5,6]]
>>> b = a.copy()
>>> id(a[0])
139979290199944
>>> id(b[0])
139979290199944
>>> a = [[1,2,3], [4,5,6]]
>>> b = copy.deepcopy(a)
>>> id(a[0])
139979230660360
>>> id(b[0])
139979230762632