Input, output and operators

print

The print accepts several values and two parameters that I use a lot: sep (separator) and end (what comes at the end, the newline by default):

print("a", "b", "c", sep="-")
print("sem quebra", end=" ")
print("de linha")

a-b-c
sem quebra de linha

input

The input shows a question, waits for the person to type, and returns always a string. If you need a number, convert:

nome = input("Qual é o seu nome? ")
idade = int(input("Quantos anos você tem? "))
print(f"Olá, {nome}. Em 5 anos você terá {idade + 5} anos.")

Qual é o seu nome? Ana
Quantos anos você tem? 30
Olá, Ana. Em 5 anos você terá 35 anos.

Never trust what the person types

If someone types trinta, the int() raises ValueError and the program crashes. In the exceptions chapter, I show how to validate the input without crashing the program.

Arithmetic operators

print(10 / 3)
print(10 // 3)
print(-7 // 2)
print(10 % 3)
print(2 ** 8)
print(divmod(17, 5))

3.3333333333333335
3
-4
1
256
(3, 2)

Be careful with -7 // 2: integer division rounds down, so the result is -4, and not -3. The divmod returns quotient and remainder at once.

Comparison and logical operators

Comparisons can be chained, just like in mathematics:

x = 7
print(1 < x < 10)
print(x == 7.0)
print("a" < "b")

True
True
True

The operators and, or and not have two characteristics that most courses do not mention. First, and and or return one of the operands, not necessarily True or False. Second, they perform short-circuiting: they stop as soon as the result is determined.

nome = "" or "anônimo"
print(nome)
print(0 and 5)
print(3 and 5)
print(not [])

anônimo
0
5
True

The language valor or padrao is common to define a default value. Use with caution: it also swaps 0 and "", which can be valid values.

Membership and identity

The operators in and not in ask if an item is in a collection. The operators is and is not ask if two names point to the same object. Use is only with None, True and False:

print("py" in "python", 3 not in [1, 2])
valor = None
print(valor is None, valor is not None)

True True
True False

Compound assignment and the walrus operator

The operators +=, -=, *=, //= and friends modify and assign at once. The operator := (called the walrus, available since Python 3.8) assigns within an expression:

total = 10
total += 5
total *= 2
total //= 4
print(total)

if (n := len("python")) > 5:
    print(f"{n} caracteres")

7
6 caracteres

When an expression mixes operators, precedence decides the order: first **, then * / // %, then + -, then comparisons, then not, and and or. I never memorize the entire table: I use parentheses whenever the order is not obvious at first glance.