Operators

Arithmetic

print(7 + 3, 7 - 3, 7 * 3)
print(7 / 3, 7 // 3, 7 % 3, 7 ** 3)

10 4 21
2.3333333333333335 2 1 343

Comparison

print(5 == 5, 5 != 5)
print(5 > 3, 5 < 3, 5 >= 5, 5 <= 4)

True False
True False True False

Comparisons can be chained, and they read the way the maths does:

age = 25
print(18 <= age < 65)

True

That is genuinely one comparison, not two joined by and, so age is only evaluated once.

Logical

and, or and not, covered in the previous chapter. The thing worth repeating is that they stop early:

def loud():
    print("this ran")
    return True

print(False and loud())
print(True or loud())

False
True

Neither call happened. loud() was never needed.

Membership

in asks whether something is inside a container:

print("a" in "cat")
print(3 in [1, 2, 3])
print("x" not in "cat")
print("name" in {"name": "Ada"})

True
True
True
True

For a dictionary, in looks at the keys, not the values.

Identity

== asks whether two values are equal. is asks whether they are the same object. They are different questions:

a = [1, 2, 3]
b = [1, 2, 3]
c = a

print(a == b)
print(a is b)
print(a is c)

True
False
True

a and b hold equal contents in two separate lists. c is another name for the very same list.

Assignment shorthands

n = 10
n += 5
n -= 3
n *= 2
n //= 4
print(n)

6

The walrus

:= assigns and produces the value, so you can capture something in the same line that tests it:

values = [1, 2, 3, 4]
if (count := len(values)) > 3:
    print(f"{count} values, which is plenty")

4 values, which is plenty

Precedence

** binds tightest, then unary minus, then * / // %, then + -, then comparisons, then not, and, or.

print(2 + 3 * 4)
print((2 + 3) * 4)
print(2 ** 3 ** 2)

14
20
512

** groups right to left, so that last line is 2 ** 9, not 8 ** 2.