I can't copy a list in python

Question

I'm trying to create a copy of a list and use it inside my function.

import copy
islands=[['a','b','c',],[]]
def transfer(thing):
    new_islands=copy.copy(islands)
    new_islands[0].remove(thing)
    return new_islands
transfer('b')
print(islands)
//output ['a','c']

I have tried other methods of copying such as new_list=old_list[:] and even creating a for loop that appends every element to an empty list, but I still can't get to separate both lists.

Sorry if this has been asked before, but I really couldn't find an answer to my problem.

use `deepcopy()` to also copy the data of the inner lists. – Patrick Artner Apr 17 '18 at 19:22 — Patrick Artner, Apr 17 '18 at 19:22
Solved my question. Thanks a lot. – Badmephisto Apr 17 '18 at 19:32 — Badmephisto, Apr 17 '18 at 19:32

TwistedSim · Accepted Answer · 2018-04-17T19:48:03.827

You need to copy the inner list as well:

islands = [['a','b','c',],[]]

def transfer(thing):  
    new_islands = [islands[0].copy(), islands[1].copy()]
    new_islands[0].remove(thing)
    return new_islands

print(transfer('b'), islands)  # [['a', 'c'], []] [['a', 'b', 'c'], []]

You can also use deepcopy to copy everything inside the list:

from copy import deepcopy

islands = [['a','b','c',],[]]
def transfer(thing):
    new_islands = deepcopy(islands)
    new_islands[0].remove(thing)
    return new_islands

score -1 · Answer 2 · answered Apr 17 '18 at 19:24

-1

Let's try to make it simpler and avoid copy at all:

islands=[['a','b','c',],[]]
def transfer(islands, thing):
    return islands[0].remove(thing)

transfer(islands, 'b')
print(islands)
//output ['a','c']

answered Apr 17 '18 at 19:24

Olzhas Arystanov

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This doesn't have the same behavior ask by the OP. But avoiding the copy is a better way to achieve this! – TwistedSim Apr 17 '18 at 19:26

I can't copy a list in python

2 Answers2