How To Think Like A Computer ITSC 1212 - Functions First Edition

Unlike strings, lists are mutable. This means we can change an item in a list by accessing it directly as part of the assignment statement. Using the indexing operator (square brackets) on the left side of an assignment, we can update one of the list items.

An assignment to an element of a list is called item assignment. Item assignment does not work for strings. Recall that strings are immutable.

Here is the same example in codelens so that you can step through the statements and see the changes to the list elements.

By combining assignment with the slice operator we can update several elements at once.

We can also remove elements from a list by assigning the empty list to them.

We can even insert elements into a list by squeezing them into an empty slice at the desired location.

One important thing that makes strings different from some other Python collection types is that you are not allowed to modify the individual characters in the collection. It is tempting to use the [] operator on the left side of an assignment, with the intention of changing a character in a string. For example, in the following code, we would like to change the first letter of greeting.

Instead of producing the output Jello, world!, this code produces the runtime error TypeError: 'str' object does not support item assignment.

Strings are immutable, which means you cannot change an existing string. The best you can do is create a new string that is a variation on the original.

The solution here is to concatenate a new first letter onto a slice of greeting. This operation has no effect on the original string.

As with strings, if we try to use item assignment to modify one of the elements of a tuple, we get an error. In fact, that’s the key difference between lists and tuples: tuples are like immutable lists. None of the operations on lists that mutate them are available for tuples. Once a tuple is created, it can’t be changed.

julia[0] = 'X'  # TypeError: 'tuple' object does not support item assignment

Check your understanding

alist = [4,2,8,6,5]
alist[2] = True
print(alist)

s = "Ball"
s[0] = "C"
print(s)