How to generate multi dimensional list in python

Question

I try to generate a multi dimensional list depending on two variables: size and dim. Both of them are initialized with user input, so I don't know their values while coding. An example with dim = 3 and size = 2 would be: [[['+', '+'], ['+', '+']], [['+', '+'], ['+', '+']]]. What I have tried so far:

import copy
fields = '+'
for i in range(dim):
    fields = [copy.copy(fields) for j in range(size)]

While this works totally fine for dim in [1, 2], it only creates references to the same list if dim > 2. Example with size = 3 and dim = 3:

>>> f
[[['+', '+', '+'], ['+', '+', '+'], ['+', '+', '+']], [['+', '+', '+'], ['+', '+', '+'], ['+', '+', '+']], [['+', '+', '+'], ['+', '+', '+'], ['+', '+', '+']]]
>>> f[0][0][0] = 'X'
>>> f
[[['X', '+', '+'], ['+', '+', '+'], ['+', '+', '+']], [['X', '+', '+'], ['+', '+', '+'], ['+', '+', '+']], [['X', '+', '+'], ['+', '+', '+'], ['+', '+', '+']]]

I know this behavior from code like f = [[['+'] * 3] * 3] * 3 and thought I could prevent it with copy.copy(), but this obviously doesn't work. I tried this with Python 3.2.2 on android (QPython 3) and with Python 2.7 on Windows, I got the same results. Please note that I don't want to use a non-standard library like numpy.

GuangshengZuo · Accepted Answer · 2017-08-22 06:17:10Z

2

You can use the copy.deepcopy(). the copy.copy() is just shallowcopy.

the difference between these two methods:

the difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):

A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.

A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

answered Aug 22, 2017 at 6:17

GuangshengZuo

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Joonatan Samuel · Accepted Answer · 2017-08-22 07:03:51Z

0

I would use numpy arrays for this giving more convenient slicing and indexing operations. Also allows for more dimensions than 2-3 and cleaner code.

import numpy as np
X = np.empty((width, height), dtype=object)

Then you can fill it via your desired method, e.g:

import itertools
for x, y in itertools.product(range(width), range(height)):
     X[x, y] = '+'

answered Aug 22, 2017 at 7:03

Joonatan Samuel

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