Loops with for

for in any iterable

The for in Python does not count: it iterates over the items of any iterable object (string, list, file, dictionary):

for letra in "abc":
    print(letra)

for fruta in ["maçã", "pera"]:
    print(fruta)

a
b
c
maçã
pera

range

When you need a sequence of numbers, use range. It accepts range(fim), range(inicio, fim) and range(inicio, fim, passo), and the end is never included. It is lazy: it does not create the list in memory. The list() below serves only to show the content:

print(list(range(5)))
print(list(range(1, 6)))
print(list(range(0, 10, 2)))
print(list(range(5, 0, -1)))

[0, 1, 2, 3, 4]
[1, 2, 3, 4, 5]
[0, 2, 4, 6, 8]
[5, 4, 3, 2, 1]

enumerate and zip

Do not write for i in range(len(lista)) to get the position. The enumerate delivers position and item together. The zip iterates two sequences in parallel:

nomes = ["Ana", "Bia", "Caio"]
for posicao, nome in enumerate(nomes, start=1):
    print(posicao, nome)

notas = [9, 8, 7]
for nome, nota in zip(nomes, notas):
    print(f"{nome}: {nota}")

1 Ana
2 Bia
3 Caio
Ana: 9
Bia: 8
Caio: 7

break, continue and else

The break ends the loop. The continue skips to the next iteration. The else of the loop is little known and very useful: it runs only if the loop finished without break.

for numero in range(1, 11):
    if numero == 3:
        continue
    if numero == 6:
        break
    print(numero)
else:
    print("terminou sem break")

1
2
4
5

The else The loop shines in searches. To know if a number is prime, look for a divisor. If you find one, break. If the loop ends without finding one, the else confirms:

numero = 17
for divisor in range(2, numero):
    if numero % divisor == 0:
        print(f"{numero} não é primo")
        break
else:
    print(f"{numero} é primo")

17 é primo

Nested loops and patterns

A loop inside another executes the inner one completely on each turn of the outer. It is the basis of tables, matrices, and patterns drawn with text:

for linhas in range(1, 5):
    print("*" * linhas)

*
**
***
****

for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i * j}", end="  ")
    print()

1 x 1 = 1  1 x 2 = 2  1 x 3 = 3  
2 x 1 = 2  2 x 2 = 4  2 x 3 = 6  
3 x 1 = 3  3 x 2 = 6  3 x 3 = 9  

Multiplying a string by an integer ("*" * 3) repeats the text, and it is the trick that avoids a second loop in many designs.