Adding values to dictionary in Python

Question

The code below:

rect_pos = {}

rect_pos_single = [[0,0], [50, 0], [50, 100], [0, 100]]

i = 0

while (i<3): 

    for j in range(4):

        rect_pos_single[j][0] += 50

    print rect_pos_single

    rect_pos[i] = rect_pos_single

    i += 1

print rect_pos

The code prints subsequent iterations of the variable "rect_pos_single". Next, I add them to dictionary "rect_pos". Keys change, but value is always the same - last iteration. I do not understand why?

Have you tried extracting the element from the dictionary, performing the addition, and then re-adding it to the dictionary? What is your expected output? And what is your current output? — aplassard
– aplassard, Commented Apr 29, 2014 at 13:14

chepner · Accepted Answer · 2014-04-29 13:18:33Z

1

This line

rect_pos_single[j][0] += 50

modifies the list in-place; that is, rect_pos_single always refers to the same list object, but you change the contents of that list.

This line

rect_pos[i] = rect_pos_single

assigns a reference to the list referenced by rect_pos_single to rect_pos[i]. Each element of rect_pos refers to the same list.

The simplest change is to assign a copy of the list to the dictionary with

rect_pos[i] = copy.deepcopy(rect_pos_single)  # Make sure you import the copy module

A deep copy is needed because rect_pos_single is a list of lists, and doing a shallow copy with rect_pos_single would simply create a new list with references to the same lists that are actually modified with rect_pos_single[j][0] += 50.

answered Apr 29, 2014 at 13:18

chepner

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