Tuples: a row that stays fixed

What you will be able to do

After this lesson you can

• make a tuple and read it by position,
• explain why a tuple cannot change, and what to do instead,
• unpack a tuple into variables,
• write a function that returns two values, and
• keep records as a list of tuples.

A row that cannot change

A tuple is like a list that is locked. You write it with round brackets, and you read it by position, as for a list. But you cannot change, add or remove items after it is made.

Why would you want that? Some values belong together and must not change by accident: the two numbers of a point, the three numbers of a colour, the day, the month and the year of a date.

The picture compares the four containers that you will know by the end of this course. Look at the tuple card for now: round brackets, an order, fixed.

Predict first, then run it. Then check these points.

• Everything you know from lists works for reading: positions, negative positions, slices, len, in, index and count. A slice of a tuple is a tuple.
• The commas make the tuple, not the brackets. 3, 4 is a tuple. The brackets only make it easier to read.
• A tuple with one item needs a comma: (5,). Without the comma, (5) is only the number 5 in brackets.
• point + (5, 6) joins two tuples into a new tuple. The old ones are not changed.

It cannot change

It looks like a loss. It is a gain. If a function receives a tuple, it cannot change your data by accident. A tuple is safe. And because it never changes, it can be a key of a dictionary, as you will see.

Unpacking

Try (3, 4): the distance is exactly 5. A tuple is also how a function hands back two answers at once.

A function that gives two answers

A function can only return one thing. But that one thing can be a tuple, which holds two or more values.

Lists of tuples, and tuples as keys

Common mistakes

• Trying to change a tuple. A TypeError. Make a new tuple.
• A one-item tuple without a comma. (5) is the number 5.
• Unpacking into the wrong number of names. A ValueError.
• Using a list where a key is needed. Use a tuple.
• Confusing () for an empty tuple with {} for an empty dictionary and [] for an empty list.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Unpack and swap. You unpack a tuple, and the checker confirms each variable.
2. Build. Open the problem Read and unpack a tuple and the problem A function with two answers.
3. Stretch. Open the problem A tuple cannot change and explain how you would "change" one item.

Remember these

• A tuple is written with round brackets and cannot be changed. The commas make it.
• x, y = point unpacks a tuple. a, b = b, a swaps two variables.
• A function can return two values as a tuple, and the caller can unpack them.
• A tuple can be a dictionary key. A list cannot.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is (5, 8, 2)[1]?

2. What happens with t = (1, 2) and then t[0] = 5?

3. What is the type of (5), and of (5,)?

4. How do you swap a and b?