9.2. 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.

Activity: CodeLens Changing an item in a list. (listReassign)

The one-th 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'] print('Edam' in cheeses) print('Brie' in cheeses)

Q-3: What is printed by the following statements?

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

• [4, 2, True, 8, 6, 5]
• Item assignment does not insert the new item into the list.
• [4, 2, True, 6, 5]
• Yes, the value True is placed in the list at index 2. It replaces 8.
• Error, it is illegal to assign
• Item assignment is allowed with lists. Lists are mutable.

Q-4: What would the following code print?

values = [3, 2, 1]
values[0] = values[1]
values[2] = values[2] + 1
print(values)

• [3, 2, 1]
• That is the original contents of values, but the contents are changed.
• [2, 0, 2]
• When you set values[0] to values[1] it makes a copy of the value and doesn't zero it out.
• [2, 2, 2]
• The value at index 0 is set to a copy of the value at index 1 and the value at index 2 is incremented.
• [2, 2, 1]
• Notice that we do change the value at index 2. It is incremented by 1.

Q-5: What is printed by the following statements?

alist = [3, 67, "cat", [56, 57, "dog"], [ ], 3.14, False]
print(57 in alist)

• True
• in returns True for top level items only. 57 is in a sublist.
• False
• Yes, 57 is not a top level item in alist. It is in a sublist.

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