Aliasing and Cloning¶
Since variables refer to objects, if we assign one variable to another, both variables refer to the same object:
(listalias1)
In this case, the reference diagram looks like this:
Because the same list has two different names, a and b, we say that it
is aliased. Changes made with one alias affect the other. In the codelens example below, you can see that a and b refer
to the same list after executing the assignment statement b = a.
(chp09_is3)
Although this behavior can be useful, it is sometimes unexpected or undesirable. In general, it is safer to avoid aliasing when you are working with mutable objects. Of course, for immutable objects, there’s no problem. That’s why Python is free to alias strings and integers when it sees an opportunity to economize.
Cloning Lists¶
If we want to modify a list and also keep a copy of the original, we need to be able to make a copy of the list itself, not just the reference. This process is sometimes called cloning, to avoid the ambiguity of the word copy.
The easiest way to clone a list is to use the slice operator.
Taking any slice of a creates a new list. In this case the slice happens to
consist of the whole list.
(chp09_is4)
Now we are free to make changes to b without worrying about a. Again, we can clearly see in codelens that a and b are entirely different list objects.