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.

Assignments in Python *do not* create new objects - an assignment merely establishes a binding between a [variable] name and an object. That should explain everything, without talking about "references". — user2864740, Nov 13 '13 at 10:56

score 91 · Accepted Answer · edited May 31 '17 at 21:50

91

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[:]

edited May 31 '17 at 21:50

Nathaniel Ford

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answered Nov 13 '13 at 10:56

yuvi

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I just spend the past 3 hours trying to debug my program... I thought I was going mad. This was the most infuriating bug I have EVER ran into. Wow... – 43.52.4D. Mar 17 '16 at 07:05
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@43.52.4D. Only it's not a bug :-) – yuvi Mar 17 '16 at 07:13
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@yuvi what if its a two dimensional array? – Sachin Singh Sep 14 '17 at 09:11
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@SachinSingh then you need to loop-copy, because a matrix is actually a list of references. Copying that list does not mean copying the references as well (which makes sense). Though that's not a very common situation (I can't think of any reason why you'd ever need to do it to be honest) – yuvi Sep 23 '17 at 13:56
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If you have a list of objects, you need to do a "Deep Copy" instead: `import copy new_list = copy.deepcopy(old_list)` – JohnB Nov 01 '18 at 15:25
you can use copy method for copy data : -> copy_list = org_list.copy() – Prajit Karande Sep 10 '19 at 10:10

Roberto · Answer 2 · 2018-11-26T16:06:52.550

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)

In other words, if you don't deep copy you will end up with: copy_list = ['y','c',org_list[2],'cap'] — Al Martins, Dec 14 '20 at 13:46

score 19 · Answer 3 · answered Nov 13 '13 at 10:55

19

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 Nov 13 '13 at 10:55

John La Rooy

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what if its a 2 dimensional array. – Sachin Singh Sep 14 '17 at 09:14
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@SachinSingh, Then you need to also copy each sublist. You should probably checkout deepcopy. https://docs.python.org/3/library/copy.html – John La Rooy Sep 14 '17 at 11:47
thanks. i saw that. But is there is another way by which i can do that i mean without using deepcopy? – Sachin Singh Sep 14 '17 at 11:50
5

Python 3.3+ provides a copy method: `copy_list = org_list.copy()` – onewhaleid Jan 17 '18 at 23:25

score 11 · Answer 4 · answered Nov 13 '13 at 10:56

11

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 Nov 13 '13 at 10:56

Andrew Jaffe

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what if its a 2 dimensional array? – Sachin Singh Sep 14 '17 at 09:13
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@SachinSingh you can use `firstList = copy.deepcopy(secondList)` – Alex Oct 25 '20 at 14:35
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@Alex Thanks a lot!! I have a 2 dimensional array and you solved my issue – Alice Mar 19 '21 at 21:17

python: changes to my copy variable affect the original variable

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