Read this for more on indexing.

2. Lists are mutable

The syntax for accessing the elements of a list is the same as for accessing the characters of a string—the bracket operator. The expression inside the brackets specifies the index. Remember that the indices start at 0:

>>> cheeses[0]


Unlike strings, lists are mutable. When the bracket operator appears on the left side of an assignment, it identifies the element of the list that will be assigned.

>>> numbers = [42, 123]

>>> numbers[1] = 5

>>> numbers

[42, 5]

The one-eth element of numbers, which used to be 123, is now 5.

Figure 10.1 shows the state diagram for cheeses, numbers and empty

state diagram

Figure10.1: State diagram.

Lists are represented by boxes with the word “list” outside and the elements of the list inside. cheeses refers to a list with three elements indexed 0, 1 and 2. numbers contains two elements; the diagram shows that the value of the second element has been reassigned from 123 to 5. empty refers to a list with no elements.

List indices work the same way as string indices:

  • Any integer expression can be used as an index.
  • If you try to read or write an element that does not exist, you get an IndexError.
  • If an index has a negative value, it counts backward from the end of the list.

The in operator also works on lists.

>>> cheeses = ['Cheddar', 'Edam', 'Gouda']

>>> 'Edam' in cheeses


>>> 'Brie' in cheeses