Strings

Indices and slices

Each character has a position, starting at zero. Negative indices count from the end. The slice [início:fim:passo] excludes the final index:

texto = "Python"
print(texto[0], texto[-1], texto[1:4], texto[::-1])

P n yth nohtyP

Strings are immutable

No operation alters an existing string. They always create a new one:

try:
    texto[0] = "J"
except TypeError as erro:
    print(erro)

novo = "J" + texto[1:]
print(novo)

'str' object does not support item assignment
Jython

Everyday methods

These are the methods I use every week. All return a new string (or another value) and leave the original intact:

frase = "  Aprender Python é divertido  "
print(frase.strip())
print(frase.strip().lower())
print(frase.strip().upper())
print(frase.strip().split())
print("-".join(["a", "b", "c"]))
print(frase.replace("Python", "Go").strip())
print(frase.find("Python"))
print(frase.strip().startswith("Aprender"))
print(frase.count("e"))
print("Python" in frase)

Aprender Python é divertido
aprender python é divertido
APRENDER PYTHON É DIVERTIDO
['Aprender', 'Python', 'é', 'divertido']
a-b-c
Aprender Go é divertido
11
True
3
True

The find returns -1 when not found. The index does the same, but raises ValueError. To decide if one string contains another, use the operator in.

f-strings

F-strings are the modern way to format text. After the colon goes the format of the value:

nome = "Ana"
preco = 1234.5
quantidade = 3
print(f"{nome} comprou {quantidade} itens")
print(f"Preço: {preco:,.2f}")
print(f"Total: {preco * quantidade:.2f}")
print(f"[{nome:>8}] [{nome:<8}] [{nome:^8}]")
print(f"{quantidade=}")

Ana comprou 3 itens
Preço: 1,234.50
Total: 3703.50
[     Ana] [Ana     ] [  Ana   ]
quantidade=3

The format :,.2f uses a comma for thousands and a period for decimals, which is the American standard. For the Brazilian format, swap the symbols:

preco = 1234.5
americano = f"{preco:,.2f}"
brasileiro = americano.replace(",", "_").replace(".", ",").replace("_", ".")
print(brasileiro)

1.234,50

For real applications, the module locale or the Babel library does this by region. For a script, the above swap solves it.

Unicode and bytes

A string is a sequence of code points Unicode. Bytes only come into play when you write to disk or send over the network, and then the encoding matters:

print(ord("A"), chr(97))
palavra = "ação"
print(len(palavra))
dados = palavra.encode("utf-8")
print(dados)
print(len(dados))
print(dados.decode("utf-8"))

65 a
4
b'a\xc3\xa7\xc3\xa3o'
6
ação

The word ação has 4 characters and occupies 6 bytes in UTF-8, because ç and ã use two bytes each. Always state the encoding explicitly when reading and writing files. I will revisit this in the file chapter.