Course 1.2Lesson 4 of 5

Choosing the right container

Python has four containers for many values. A list keeps an order, can change, and allows repeats. A tuple keeps an order, cannot change, and allows repeats. A set has no order, allows no repeats, and gives a fast yes or no test. A dictionary finds a value by its key, and the keys are unique. To choose, ask three questions: do I need an order or a position, do I need to change it, and do I look things up by a name. You can change one container into another with list, tuple, set and dict, and with items.

about 25 min1 simulation2 practice questions

Lesson 4 of 5Show all lessons

Try the code and the simulations beside the lesson.

What you will be able to do

After this lesson you can

say what each of the four containers is good at,
choose the right container by asking three questions,
change one container into another, and
explain your choice in a sentence.

Four containers

You now know all four ways in which Python keeps many values together.

	List	Tuple	Set	Dictionary
Written with	`[1, 2]`	`(1, 2)`	`{1, 2}`	`{"a": 1}`
Order	yes	yes	no	order of adding
Can change	yes	no	yes	yes
Repeats	allowed	allowed	no	keys are unique
Find by	position	position	membership	key
Typical use	a sequence that grows	a fixed record	different values, overlap	a lookup table

The same data, in four containers

Predict first, then run it. Each container answers one kind of question best.

The marks in the list are read by position: "the second test".
The tuple (3, 4) is a fixed pair: x and y.
The set of names answers "is Ravi there?" fast, and nothing else.
The dictionary answers "what is the mark of Ravi?".

At the end, the program changes one container into another.

list(seen) is a list in no particular order. sorted(seen) is a list in order.
tuple(marks_list) freezes a list.
set(marks_list) drops the repeats.
list(marks_dict.items()) gives a list of (key, value) tuples. And dict(that_list) goes back the other way.

How to choose: three questions

Do I find things by position, or by a name? By a name: a dictionary. By a position: a list or a tuple.
Does it need to change? If it must stay fixed (a point, a date, a key): a tuple. If it grows or changes: a list.
Do I care about the order and the repeats? If only about what is there, and each value once: a set.

Some examples.

The job	The container	Why
the marks of a class, in the order of the roll numbers	list	an order, and it grows
the (latitude, longitude) of a city	tuple	two values that belong together
the different cities that you visited	set	each once, and you ask "did I go there?"
the population of each city	dictionary	found by the name of the city
the words of a sentence, to count them	list, then a dictionary	split gives a list, counting needs a dictionary

For a handful of items the difference does not matter. For thousands of lookups in a big collection, a set wins.

Speed in one sentence

For a big collection, x in a_set and d[key] are fast. x in a_list is slow, because Python looks at the items one by one. If you test membership many times, a set is the better tool.

Common mistakes

Using a list for membership tests in a big loop. Slow. Use a set.
Using a dictionary where a list is enough, with the numbers 0, 1, 2 as keys.
Using a list where the data must not change. A tuple is safer.
Using a set and expecting an order.
Forgetting that set(a_list) loses the repeats and the order.

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:

Warm-up. Open the lab Choose and convert. You convert between containers, and the checker confirms the results.
Build. Open the problem Which container? from the dictionaries course, if you did not, and the problem Which container fits?
Stretch. Take five facts about yourself and your family, and choose a container for each, with a sentence about why.

Remember these

List: an order, can change. Tuple: an order, fixed. Set: no order, no repeats. Dictionary: found by key.
Ask three questions: position or name, fixed or changing, order and repeats.
list(), tuple(), set() and dict() change one container into another.
in and a dictionary lookup are fast for sets and dictionaries.