Tuples and sets

Tuples

A tuple is a sequence immutable. Use it for data that forms a fixed record (coordinates, pairs of values, function returns):

dias = ("seg", "ter", "qua")
print(dias[0], len(dias))
try:
    dias[0] = "dom"
except TypeError as erro:
    print(erro)

seg 3
'tuple' object does not support item assignment

A syntax trick: what forms the tuple is the comma, not the parenthesis. A tuple with one element needs the final comma:

um = (1,)
nao_tupla = (1)
print(type(um), type(nao_tupla))

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

Unpacking is where the tuple appears the most. The * captures "the rest" in a list:

ponto = (3, 4)
x, y = ponto
primeiro, *resto = [10, 20, 30, 40]
print(x, y)
print(primeiro, resto)

3 4
10 [20, 30, 40]

When you want a tuple with names in the fields, use namedtuple:

from collections import namedtuple

Ponto = namedtuple("Ponto", ["x", "y"])
p = Ponto(3, 4)
print(p, p.x + p.y)

Ponto(x=3, y=4) 7

The immutability of the tuple is shallow: it ensures that the items are not swapped, but if an item is mutable, it can still change:

registro = ("ana", [1, 2])
registro[1].append(3)
print(registro)

('ana', [1, 2, 3])

Sets

A set holds uniqueitems, with no guaranteed order, and answers "is this item here?" in practically constant time. The operations follow set theory:

s = {1, 2, 2, 3, 3, 3}
print(s, len(s))
vazio = set()
print(type({}), type(vazio))
a = {1, 2, 3, 4}
b = {3, 4, 5, 6}
print(a | b, a & b, a - b, a ^ b)
print(3 in a)

{1, 2, 3} 3
<class 'dict'> <class 'set'>
{1, 2, 3, 4, 5, 6} {3, 4} {1, 2} {1, 2, 5, 6}
True

{} creates a dictionary empty, not a set. For an empty set, use set().

The items in a set need to be hashable, which in practice means immutable. A list cannot be included, but a tuple can. The frozenset is the immutable version of the set:

try:
    {[1, 2]}
except TypeError as erro:
    print(erro)

congelado = frozenset([1, 2])
print(congelado)

unhashable type: 'list'
frozenset({1, 2})

The order of a set is not guaranteed

A set has no position, so there is no s[0]. The order in which it prints may change between executions, especially with strings. If you need order, use sorted(conjunto).

Which structure to use