10.2: Lists are mutable
- Page ID
- 40781
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] 'Cheddar'
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 \(\PageIndex{1}\) shows the state diagram for cheeses
, numbers
and empty
:

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 True >>> 'Brie' in cheeses False