python: changes to my copy variable affect the original variable

Question

I’ve got a list that I create a copy of in order to do some manipulations while still keeping the original list. However, when I set copy_list equal to org_list, they become the same thing, and if I change copy_list, org_list changes too. For example:

org_list = ['y', 'c', 'gdp', 'cap']

copy_list = org_list

copy_list.append('hum')

print(copy_list)
print(org_list)

returns

['y', 'c', 'gdp', 'cap', 'hum']
['y', 'c', 'gdp', 'cap', 'hum']

I don’t know too much about what is actually going on but it looks like org_list is actually passing itself to copy_list so that they are actually the same thing.

Is there a way to make an independent copy of org_list without doing something clumsy like:

copy_list = []
for i in org_list:
    copy_list.append(i)

I say this because I have the same problem with other types of variables, for example a pandas dataframe.

Asked By: natsuki_2002

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Source

Answer 1

This is just copying the reference

copy_list = org_list

you should use

copy_list = org_list[:]    # make a slice that is the whole list

or

copy_list = list(org_list)

Answered By: John La Rooy

Answer 2

That is because in python setting a variable actually sets a reference to the variable. Almost every person learning python encounters this at some point. The solution is simply to copy the list:

copy_list = org_list[:]

Answered By: yuvi

Answer 3

Variable names in python are references to the original. To actually make a copy, you need to be explicit:

import copy

copy_list = copy.copy(org_list)

Answered By: Andrew Jaffe

Answer 4

When you write

org_list = ['y', 'c', 'gdp', 'cap']

you create the list object, and give it the name “org_list”.

Then when you do

copy_list = org_list

you just mean, “the name copy_list refers to the same object as org_list does”.

If your list only contains immutable types, then you can create a copy by

copy_list = list(org_list)

But note that this is only valid if the list objects are immutable, because it creates a SHALLOW copy, i.e. the list is copied, but every element on the list is not duplicated.

If you have i.e. a list of lists and want EVERYTHING to be duplicated, you need to perform a DEEP copy:

import copy
org_list = ['y', 'c', ['gdp', 'rtd'], 'cap']
copy_list = copy.deepcopy(org_list)

Answered By: Roberto

python: changes to my copy variable affect the original variable

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