Lesson 07 · Collections and functions

Lists and tuples

A list keeps many values in order under one name, and you can change it: add items, replace them, read them back by index. A tuple keeps values in order too, but once it is made it cannot change. Step through a six-line program, lists.py, that builds a list, grows it with append(), changes one slot, prints from it, then makes a tuple and watches Python refuse to change it.

12 min8 steps5 questionsBeginner

Fig 7 · Interactive

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5 quick questions

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01After colors = ["red", "green", "blue"], what is colors[1]?

Indexes count from 0, so colors[0] is 'red' and colors[1] is the second item, 'green'.

02nums = [4, 7] then nums.append(9). What are nums and len(nums) now?

append() adds one item to the end of the list, so nums becomes [4, 7, 9] and has 3 items.

03What does ["a", "b", "c"][-1] give?

A negative index counts from the end: -1 is the last item, 'c'.

04point = (3, 4). What happens on point[0] = 9 ?

A tuple is immutable: its items cannot be replaced. To get (9, 4), make a new tuple: point = (9, 4).

05Which is the better fit for a tuple rather than a list?

A tuple suits a small, fixed group of values that belong together, such as the parts of a date. Things that grow or change suit a list.

Key terms

Glossary

List: An ordered collection of values in square brackets, such as ["apple", "fig"]. It can change after it is made.
Tuple: An ordered collection of values, usually in round brackets, such as (3, 4). It cannot change after it is made.
Item: One value inside a list or tuple.
Index: An item's position, counted from 0. fruits[0] is the first item; fruits[-1] is the last.
len(): A built-in function that gives the number of items, such as len(["a", "b"]) == 2.
append(): A list method that adds one item to the end of the list.
Mutable / immutable: Mutable values (lists) can be changed in place; immutable values (tuples, strings, numbers) cannot.
TypeError: The error Python raises when a value does not support an operation, such as assigning to an item of a tuple.

Fine print

Notes and sources

In the boxes and at the >>> prompt, Python writes strings with single quotes, such as 'fig'; print() writes them without quotes.
In the terminal, the figure shortens the traceback: the real one also names the file and line (line 6) and repeats the failing line with ~~~~~^^^ under it. The last line is exactly as Python prints it.
An error like this stops the program at that line: lines 1 to 5 have already run, which is why fig 3 is printed first.
An index must point at an existing slot: with three items, fruits[3] raises IndexError: list index out of range.
It is the comma that makes a tuple; the round brackets are usually optional. A one-item tuple needs a trailing comma, (5,), because (5) is just the number 5.

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