Modifying Lists

Suppose you’re typing in a list of the noble gases and your fingers slip:

The error here is that you typed ’none’ instead of ’neon’. Here’s the memory model that was created by that assignment statement:

Rather than retyping the whole list, you can assign a new value to a specific element of the list:

Here is the result after the assignment to nobles[1]:

That memory model also shows that list objects are mutable. That is, the contents of a list can be mutated.

In the previous code, nobles[1] was used on the left side of the assignment operator. It can also be used on the right side. In general, an expression of the form L[i] (list L at index i) behaves just like a simple variable (see ​Variables and Computer Memory: Remembering Values​).

If L[i] is used in an expression (such as on the right of an assignment statement), it means “Get the value referred to by the memory address at index i of list L.”

On the other hand, if L[i] is on the left of an assignment statement (as in nobles[1] = ’neon’), it means “Look up the memory address at index i of list L so it can be overwritten.”

In contrast to lists, numbers and strings are immutable. You cannot, for example, change a letter in a string. Methods that appear to do that, like upper, actually create new strings:

Because strings are immutable, it is only possible to use an expression of the form s[i] (string s at index i) on the right side of the assignment operator.