Casting

Converting on purpose

Python will not quietly turn a string into a number for you. You ask, using the type's own name as a function:

print(int("42") + 1)
print(float("3.5") * 2)
print(str(99) + " bottles")

43
7.0
99 bottles

Why you need it

Anything typed by a person arrives as text, even when it looks like a number:

answer = "10"
print(answer * 3)
print(int(answer) * 3)

101010
30

Multiplying a string repeats it. That is a real feature, and it is also the classic bug when you forgot to convert.

int() truncates

Going from float to int throws the fraction away. It does not round:

print(int(3.9))
print(int(-3.9))
print(round(3.9))

3
-3
4

When conversion fails

int() accepts a string only if the whole thing is a number. Anything else raises ValueError:

print(int("42"))
print(int(" 42 "))

42
42

Surrounding whitespace is fine. A decimal point is not — int("3.5") fails, so convert in two steps:

print(int(float("3.5")))

3

Handling bad input

Since failure is an exception, wrap the conversion when the value comes from outside your program:

for text in ["12", "twelve"]:
    try:
        print("got", int(text))
    except ValueError:
        print(text, "is not a number")

got 12
twelve is not a number

Converting to bool

bool() follows the truthiness rules: empty and zero are False, everything else is True.

print(bool(0), bool(1), bool(-1))
print(bool(""), bool("hi"), bool("False"))
print(bool([]), bool([0]))

False True True
False True True
False True

Other bases

int() takes a second argument saying what base the text is written in:

print(int("ff", 16))
print(int("1010", 2))

255
10