Type conversion

Sometimes you have a value of one type and need another, such as text from the keyboard that should be a number. Converting a value is called type conversion (or casting).

The conversion functions

age_text = "21"
age = int(age_text)
print(age + 1)

price = float("19.99")
print(price * 2)

label = "Score: " + str(87)
print(label)

int() cuts off, it doesn't round

Converting a float to an int drops everything after the decimal point:

print(int(3.99))
print(int(-3.99))
print(round(3.99))   # use round() if you want the nearest whole number

Conversions that fail

If the text does not look like a number, Python raises a ValueError:

print(int("twelve"))

Also note that int("3.5") fails: int() only accepts whole-number text. Use int(float("3.5")) if you need it.

What counts as True or False

bool() follows a simple rule: empty or zero is False, everything else is True.

print(bool(0), bool(0.0), bool(""), bool([]), bool(None))
print(bool(1), bool(-5), bool("hi"), bool([0]), bool(" "))

Yes, bool(" ") is True: a string containing a space is not empty.

Automatic conversion

Python converts automatically in a few safe cases. Mixing an int and a float in arithmetic gives a float:

print(5 + 2.0)
print(type(5 + 2.0))

But it never silently turns text into numbers or numbers into text; that is why "Age: " + 21 is an error rather than a guess.