Tuples

A shopping list gets edited all day. A birth certificate is printed once, and nobody crosses things off it.

Lists are wonderful because they can change: you add, remove, and sort items whenever you like. But some collections shouldn't change. The coordinates of a place on a map. The red, green, and blue values of a color. A date made of a year, a month, and a day. If one of those changed by accident, the whole thing would be wrong.

For fixed collections like these, Python has the tuple.

A printed certificate

Compare a shopping list with a printed birth certificate. Both hold several pieces of information in order. But the shopping list is meant to be edited, while the certificate is meant to be read. If the date on a certificate needs fixing, you don't scribble on it; you get a new certificate.

A tuple is the certificate: an ordered collection, like a list, that can never be changed once it's made. The fancy word for that is immutable, the same as strings.

Making a tuple

Write the items between parentheses, separated by commas:

point = (3, 4)
color = (255, 128, 0)
birthday = (2009, 6, 15)

print(point)
print(len(color))

(3, 4)
3

It's actually the commas that make a tuple, not the parentheses. The parentheses just make it easier to see. That leads to one odd case: a tuple with only one item needs a trailing comma, or Python thinks the parentheses are just grouping a single value:

not_a_tuple = (5)
one_item = (5,)
print(type(not_a_tuple))
print(type(one_item))

<class 'int'>
<class 'tuple'>

Reading a tuple

Everything that reads a list works on a tuple too: indexes, negative indexes, slices, len, in, and for loops:

days = ("Mon", "Tue", "Wed", "Thu", "Fri")
print(days[0], days[-1])
print(days[1:3])
print("Sat" in days)
for day in days[:2]:
    print("School day:", day)

Mon Fri
('Tue', 'Wed')
False
School day: Mon
School day: Tue

...but never changing it

What a tuple doesn't have is any way to change it. There's no append, no remove, no sort, and no assigning to an index:

point = (3, 4)
point[0] = 10  # error: TypeError: 'tuple' object does not support item assignment

That's not a limitation to work around; it's the whole point. When you pass a tuple to a function, or store it somewhere, you know for certain it will come back exactly the same.

If you do need a changed version, build a new tuple, or convert to a list, change that, and convert back:

point = (3, 4)
moved = (point[0] + 1, point[1])
print(moved)

as_list = list(point)
as_list.append(0)
print(tuple(as_list))

(4, 4)
(3, 4, 0)

Unpacking

Tuples really shine with unpacking: splitting a tuple into separate variables in one line, one variable per item:

birthday = (2009, 6, 15)
year, month, day = birthday
print(f"Born on day {day} of month {month}, {year}")

Born on day 15 of month 6, 2009

You've been doing this already, without the name. x, y = 3, 4 from Variables and Naming creates the tuple (3, 4) and unpacks it at once. And in Parameters and Return Values, a function that returned each, left was really returning a tuple:

def share(cookies, friends):
    return cookies // friends, cookies % friends

result = share(20, 6)
print(result)
print(type(result))

each, left = share(20, 6)
print(each, left)

(3, 2)
<class 'tuple'>
3 2

The number of variables must match the number of items, or Python tells you:

year, month = (2009, 6, 15)  # error: ValueError: too many values to unpack (expected 2)

Unpacking in loops

Unpacking works in a for loop too, which makes lists of tuples very pleasant to work with. Each tuple below is one student's name and score:

results = [("Maria", 91), ("Ben", 78), ("Carlo", 95)]

for name, score in results:
    print(f"{name}: {score}")

Maria: 91
Ben: 78
Carlo: 95

This is how enumerate from Lists works: it hands out (position, item) tuples, and for position, name in ... unpacks them.

Keeping each student's name and score together in one tuple is safer than two separate lists kept in step by index. Remove a tuple, and the name and score leave together.

List or tuple?

• Use a list for a collection of similar things that may grow, shrink, or change: students in a class, items in a cart, scores so far.
• Use a tuple for a small, fixed group of values that belong together as one thing: a point, a color, a date, a name and a score.

A handy test: if you'd describe it as "a list of..." it's a list. If you'd describe it as "a ... made of three parts," it's a tuple.

Try it

This program describes a school trip using tuples: a start point, stops along the way, and a meeting time. Predict every line.

** 0.5 means "to the power of one half," which is a square root. And :02d pads the minute with a zero, so 30 stays 30 but 5 would become 05.

Running Python here

Running Python right in the browser is coming to Syntaxia soon. For now, you can read and edit the code here. If you have Python installed on your computer (see Setting Up), save it as main.py and run it with python main.py (or python3 main.py on macOS and Linux).

Check your prediction

Meet at 14:30
Museum is 5.0 km from school
Park is 10.0 km from school
Library is 6.3 km from school
Last stop: Library
Stops with x = 6: ['Park', 'Library']
Single item: <class 'tuple'> <class 'str'>

Each stop is a tuple of three things, unpacked into name, x, y in the loop. The distances come from the Pythagorean theorem: the Museum at (3, 4) is exactly 5 km away. The line with square brackets is a list comprehension, which you'll meet in two lessons' time. And only the version with a trailing comma is a tuple.

Try it yourself

1. Add a fourth stop, ("Zoo", 0, 5). How far is it from school?
2. Try to change meet_time[0] to 15. What error do you get? Then make a new tuple for 3:30 PM instead.
3. Write a function midpoint(a, b) that takes two point tuples and returns the point halfway between them, as a tuple.

Check your understanding

What is the main difference between a tuple and a list?

A tuple can only hold numbers

A tuple cannot be changed after it is made

Tuples have no order

Lists cannot be looped over

Which of these is a tuple with one item?

(5)

[5]

(5,)

{5}

point = (3, 4). How do you put 3 in x and 4 in y in one line?

x, y = point

x = y = point

(x + y) = point

point = x, y

Up next

Lists and tuples find things by position: item 0, item 1, item 2. But often you want to look something up by name: a student's score, a word's meaning, a product's price. For that, Python has Dictionaries.