Update list in python

Question

I'm a fresher in python. I have some troubles with list. After creating the list, i update it. it's right when I print each element of list in loop wrong , but wrong when I return list: all elements of list are same

def shift(array,n):
    for i in range(n):
        array.append(array[0])
        array.pop(0)
    return array

def shiftKey(array):
    array_shift = []
    for i in range(16):
        array_shift.append([])

    array_shift[0] = [shift(array[0],1),shift(array[1],1)]
    print (array_shift[0])
    for j in range(1,16):
        if j == 1 or j == 8 or j == 15:
            a1 = shift(array_shift[j-1][0],1)
            a2 = shift(array_shift[j-1][1],1)
            array_shift[j] = [a1,a2]
            print (array_shift[j])
        else:
            a1 = shift(array_shift[j-1][0],2)
            a2 = shift(array_shift[j-1][1],2)
            array_shift[j] = [a1,a2]
            print (array_shift[j])
    return array_shift
shiftKey([[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16],[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]])

and this's the result expected

[[[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1], [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1]] [[3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2], [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2]] [[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4], [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4]] [[7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6], [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6]] [[9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8], [9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8]] [[11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10], [11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]] [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]] [[15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14], [15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]] [[16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15], [16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]] [[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1], [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1]] [[4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3], [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3]] [[6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5], [6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5]] [[8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7], [8, 9, 10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7]] [[10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9], [10, 11, 12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9]] [[12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11], [12, 13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]] [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]]]

but it's print like below when i print list

[[[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12],
[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12],
[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]], [[13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [13, 14, 15, 16, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]]]

There are to major classes of objects in Python, immutable (strings, ints, and tuples, ...) and mutables (list, dicts, ...). Lists are mutable which sort of act like pass by reference. When you are appending lists to your list of lists, you are not actually creating new lists. You are making new pointers to the same list object. You are printing as you are going through the updates, so you see each change occur. When you return the final object, all you have is a list with many pointers, all pointing to the same list object, which is at the place you left it after the last operation. — ericstevens26101
– ericstevens26101, Commented Apr 28, 2020 at 8:36
Hi, can you edit your test case to show a minimal reproducible example please? — Pedro Trujillo
– Pedro Trujillo, Commented Apr 28, 2020 at 16:47

Ketan Krishna Patil · Accepted Answer · 2020-04-28 09:00:50Z

1

You need to use deepcopy functionality so you pass a copy of list instead of reference of the list.

Also, you can use the following easy function for rotation:

def shift(input_list, rotate):
    return input_list[rotate:] + input_list[:rotate]

I will suggest you read about defaultlist.

I corrected my code and gave an alternate solution, please try this one.

import copy

def shift(array,n):
    for i in range(n):
        array.append(array[0])
        array.pop(0)
    return array

def shiftKey(array):
    array_shift = []
    array_shift.append([shift(array[0],1),shift(array[1],1)])
    print (array_shift[0])
    for j in range(1,16):
        if j == 1 or j == 8 or j == 15:
            a1 = shift(copy.deepcopy(array_shift[j-1][0]),1)
            a2 = shift(copy.deepcopy(array_shift[j-1][1]),1)
            array_shift.append([a1,a2])
            print (array_shift[j])
        else:
            a1 = shift(copy.deepcopy(array_shift[j-1][0]),2)
            a2 = shift(copy.deepcopy(array_shift[j-1][1]),2)
            array_shift.append([a1,a2])
            print (array_shift[j])
    return array_shift

shiftKey([[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16],[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]])

edited Apr 28, 2020 at 9:00

answered Apr 28, 2020 at 8:28

Ketan Krishna Patil

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2 Comments

justinNg Over a year ago

Thanks.It's working, but the first element is wrong

Ketan Krishna Patil Over a year ago

Try solution posted above.

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