Cloning a List in Python

Date April 14, 2008 by Isaac

As far as I know there is no official way to clone a list in Python. This is desirable at times because normal variable assignment of lists is just a reference copy. For example:

 
>>> listA = ['A', 'B', 'C', 'D']
>>> listB = listA
>>> listA
['A', 'B', 'C', 'D']
>>> listB
['A', 'B', 'C', 'D']
>>> listA.remove('C')
>>> listA
['A', 'B', 'D']
>>> listB
['A', 'B', 'D']
 

As you can see, B was assigned to reference the same list as A and removing something from A also removed it from B.

At times that is fine, however, sometimes you need a clone of the list so that changes in A do not reflect in B. A very simple way to accomplish that is by using slicing. Our example now becomes:

 
>>> listA = ['A', 'B', 'C', 'D']
>>> listB = listA[:]
>>> listA
['A', 'B', 'C', 'D']
>>> listB
['A', 'B', 'C', 'D']
>>> listA.remove('C')
>>> listA
['A', 'B', 'D']
>>> listB
['A', 'B', 'C', 'D']
 

There we go, problem solved.

Category Posted in Python

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