Numbers

int and float

An int is a whole number. A float has a decimal point. Writing the point is what makes the difference:

whole = 7
decimal = 7.0
print(type(whole), type(decimal))

<class 'int'> <class 'float'>

Python integers have no size limit. They grow to fit:

print(2 ** 100)

1267650600228229401496703205376

Arithmetic

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

10
4
21
2.3333333333333335

Note the last one: / always gives a float, even when the division is exact.

print(6 / 3)
print(type(6 / 3))

2.0
<class 'float'>

Floor division and remainder

// divides and throws away the fraction. % gives what is left over:

print(7 // 3)
print(7 % 3)
print(-7 // 3)
print(-7 % 3)

2
1
-3
2

% is the usual way to ask "is this even?" or "every nth time":

for n in range(6):
    if n % 2 == 0:
        print(n, "is even")

0 is even
2 is even
4 is even

Powers

print(3 ** 2)
print(2 ** 0.5)

9
1.4142135623730951

Rounding

print(round(3.7))
print(round(3.14159, 2))

4
3.14

print(round(0.5), round(1.5), round(2.5))

0 2 2

Floats are approximations

A float is stored in binary, and some decimal fractions have no exact binary form. The result is the most reported "bug" in every language that uses them:

print(0.1 + 0.2)
print(0.1 + 0.2 == 0.3)

0.30000000000000004
False

Nothing is broken. 0.1 was never exactly a tenth. Compare with a tolerance instead of ==:

from math import isclose
print(isclose(0.1 + 0.2, 0.3))

True

For money, use decimal, which counts in base ten:

from decimal import Decimal
print(Decimal("0.1") + Decimal("0.2"))

0.3

Readable long numbers

Underscores in a numeric literal are ignored, so you can group digits:

population = 8_100_000_000
print(population)

8100000000