Tuples and Sets

Lists and dictionaries do most of the work, but two smaller tools complete the everyday kit, each with one clear job. A tuple is a small, fixed group of values that belong together, such as a pair of coordinates. A set is a collection of unique values, the right tool for "have I seen this before?".

Think of a tuple as a sealed pack of three batteries: the pack is one thing, and you cannot swap one battery out. A set is like a guest book where each name is written only once, however many times that guest visits.

Tuples

A tuple is written in round brackets, with commas between the values. You read items by position, exactly as with a list. What you cannot do is change it: assigning to a position is a TypeError, because a tuple does not support item assignment. That fixedness is the point. A pair of coordinates, a date split into year, month and day, a name and its score: things whose parts belong together and should not drift apart.

>>> position = (48.86, 2.35)
>>> position[0]
48.86
>>> position[0] = 50.0
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    position[0] = 50.0
    ~~~~~~~~^^^
TypeError: 'tuple' object does not support item assignment

Unpacking

Unpacking hands the values of a tuple, or a list, to several names at once. latitude, longitude = position gives the first value to the first name and the second to the second. The number of names must match the number of values. This is exactly what happened in the last topic, where each pair from items was unpacked into a name and a number.

>>> latitude, longitude = position
>>> latitude
48.86
>>> longitude
2.35

Sets

A set holds each value at most once. Write it in curly braces without colons, or turn a list into a set with set. Duplicates disappear, which makes a set the quickest way to find the distinct values in a list. Checking whether a value is in a set with in is instant, even for a set of a million items. A set keeps no particular order, so do not rely on the order it prints in.

>>> visitors = ["Anna", "Omar", "Anna", "Lena", "Omar"]
>>> unique = set(visitors)
>>> len(unique)
3
>>> "Lena" in unique
True

One Trap: Empty Braces

A pair of empty curly braces makes an empty dictionary, not an empty set, because dictionaries were in Python first. To make an empty set, write set() with nothing in the brackets.

>>> type({})
<class 'dict'>
>>> type(set())
<class 'set'>

Choosing

A list when you have an ordered collection that changes. A tuple when you have a fixed group of values that belong together. A set when what matters is whether something is present, and each thing counts once. A dictionary when you look things up by name.