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Engineering LibreTexts

8.2: 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:

>>> print(cheeses[0])
Cheddar

Unlike strings, lists are mutable because you can change the order of items in a list or reassign an item in a list. When the bracket operator appears on the left side of an assignment, it identifies the element of the list that will be assigned.

 

>>> numbers = [17, 123]
>>> numbers[1] = 5
>>> print(numbers)
[17, 5]

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

 

You can think of a list as a relationship between indices and elements. This relationship is called a mapping; each index "maps to" one of the 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