Python lists don't change values correctly when assigned to to other lists

Question

I encountered an error in my Python 3 code, and after intense debugging, I found out that Python doesn't appear to assign lists correctly:

$ python3
Python 3.4.2 (v3.4.2:ab2c023a9432, Oct  5 2014, 20:42:22) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> test_1 = [12, 34]
>>> test_1
[12, 34]
>>> test_2 = [12, 34]
>>> test_2
[12, 34]
>>> test_2 = test_1
>>> test_2
[12, 34]
>>> test_1[0] = 'This changes in both arrays!'
>>> test_1
['This changes in both arrays!', 34]
>>> test_2
['This changes in both arrays!', 34]
>>>

Why is this happening? Is this intended? How do I stop it from happening???

Telz · Accepted Answer · 2014-12-14 20:14:58Z

1

When you do test_2 = test_1 you are making test_2 point to the list pointed by test_1.

You could do instead:

>>> test_2 = test_1.copy()

answered Dec 14, 2014 at 20:14

Telz

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1 Comment

inspectorG4dget Over a year ago

or test_2 = test_1[:]

Behram Mistree · Accepted Answer · 2014-12-14 20:16:53Z

1

This is expected behavior. Python lists pass by reference. This means that when you assign a list, instead of making a copy of the list and assigning this new list to the new variable, it has both variables point at the same underlying list. This can be useful in a lot of cases. But it seems like you want to actually copy the list. To do so, do the following:

test_2 = list(test_1)

answered Dec 14, 2014 at 20:16

Behram Mistree

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