Tuples and Sets

Lists are not the only way to group values. Tuples and sets each solve a slightly different problem.

Tuples

A tuple is like a list that can't be changed after you create it. Use parentheses:

point = (3, 4)
colors = ("red", "green", "blue")
empty = ()

A tuple with only one item needs a trailing comma, otherwise Python treats the parentheses as plain grouping:

single = (5,)    # a tuple
not_tuple = (5)  # just the number 5

Indexing and unpacking

Indexing and slicing work the same as in lists:

print(colors[0])    # red
print(colors[-1])   # blue
print(colors[:2])   # ('red', 'green')

The most useful tuple trick is unpacking: assigning each item to its own variable in one line.

point = (3, 4)
x, y = point
print(x)   # 3
print(y)   # 4

The number of variables must match the number of items. Unpacking also gives you a tidy way to swap two values:

a = 1
b = 2
a, b = b, a
print(a, b)   # 2 1

Python quietly uses tuples all over the place. When you write return low, high in a function, you're returning a tuple.

Sets

A set is an unordered collection of unique items. Use curly braces:

colors = {"red", "green", "blue"}
print(colors)   # the order may vary

Key facts:

• A set never contains duplicates. Adding something that is already there does nothing.
• Sets have no order, so there are no indexes. colors[0] is an error.
• To make an empty set, use set(). A bare {} is an empty dictionary, which you'll meet next.

A very common use is removing duplicates by converting a list into a set:

numbers = [1, 2, 2, 3, 3, 3]
unique = set(numbers)
print(unique)        # {1, 2, 3}
print(len(unique))   # 3

Changing a set:

s = {1, 2}
s.add(3)        # {1, 2, 3}
s.remove(2)     # {1, 3}, but raises an error if 2 isn't there
s.discard(99)   # does nothing if 99 isn't there
print(3 in s)   # True

Looping over a set works, but the order isn't guaranteed. Use sorted(s) when you need a predictable order. It gives you a sorted list.

Set operations

Sets support the same operations as in math class. Say a = {1, 2, 3, 4} and b = {3, 4, 5}:

When to use which

Now try it yourself. The next step is three puzzles that use sets and tuples.