[FIXED] How do I clone a list so that it doesn't change unexpectedly after assignment?

Issue

While using new_list = my_list, any modifications to new_list changes my_list every time. Why is this, and how can I clone or copy the list to prevent it?

Solution

new_list = my_list doesn't actually create a second list. The assignment just copies the reference to the list, not the actual list, so both new_list and my_list refer to the same list after the assignment.

To actually copy the list, you have several options:

• You can use the builtin list.copy() method (available since Python 3.3):

new_list = old_list.copy()

• You can slice it:

new_list = old_list[:]

Alex Martelli's opinion (at least back in 2007) about this is, that it is a weird syntax and it does not make sense to use it ever. ;) (In his opinion, the next one is more readable).

• You can use the built in list() function:

new_list = list(old_list)

• You can use generic copy.copy():

import copy
new_list = copy.copy(old_list)

This is a little slower than list() because it has to find out the datatype of old_list first.

• If you need to copy the elements of the list as well, use generic copy.deepcopy():

import copy
new_list = copy.deepcopy(old_list)

Obviously the slowest and most memory-needing method, but sometimes unavoidable. This operates recursively; it will handle any number of levels of nested lists (or other containers).

Example:

import copy

class Foo(object):
    def __init__(self, val):
         self.val = val

    def __repr__(self):
        return f'Foo({self.val!r})'

foo = Foo(1)

a = ['foo', foo]
b = a.copy()
c = a[:]
d = list(a)
e = copy.copy(a)
f = copy.deepcopy(a)

# edit orignal list and instance 
a.append('baz')
foo.val = 5

print(f'original: {a}\nlist.copy(): {b}\nslice: {c}\nlist(): {d}\ncopy: {e}\ndeepcopy: {f}')

Result:

original: ['foo', Foo(5), 'baz']
list.copy(): ['foo', Foo(5)]
slice: ['foo', Foo(5)]
list(): ['foo', Foo(5)]
copy: ['foo', Foo(5)]
deepcopy: ['foo', Foo(1)]

Answered By - Felix Kling

Answer Checked By - Katrina (PHPFixing Volunteer)