Type conversion in Python

Type conversion means making a value of one type from a value of another type, for example the number 42 from the text "42". Python does this when you call the type's name as a function: int(), float(), str() or bool().

The program

print(int("42") + 1)
print(float("3.5") * 2)
print(str(7) + " days")

print(int(9.99))
print(int(-9.99))
print(float(5))

print(bool(0), bool(3), bool(""), bool("no"))

43
7.0
7 days
9
-9
5.0
False True False True

How it works

• int("42") reads text as a whole number. float("3.5") reads text as a number with a decimal point.
• str(7) turns a number into text, so it can be joined to other text with +.
• int() on a float removes the decimal part. It does not round: int(9.99) is 9, and int(-9.99) is -9.
• bool() gives False for 0, 0.0, None and empty containers such as "", [] and {}, and True for almost everything else. So bool("no") and even bool("False") are True, because the string is not empty.

What fails

int() accepts only text that looks like a whole number. Spaces at the start and end are allowed, but a decimal point, a comma or a word is not:

for text in ["42", " 42 ", "3.5", "4,000", "seven"]:
    try:
        print(repr(text), "->", int(text))
    except ValueError as error:
        print(repr(text), "-> ValueError:", error)

'42' -> 42
' 42 ' -> 42
'3.5' -> ValueError: invalid literal for int() with base 10: '3.5'
'4,000' -> ValueError: invalid literal for int() with base 10: '4,000'
'seven' -> ValueError: invalid literal for int() with base 10: 'seven'

To get a whole number from "3.5", convert in two steps, and remove the comma from "4,000" first:

print(int(float("3.5")))
print(int("4,000".replace(",", "")))

3
4000

int(float(...)) cuts off the decimal part. Use round(float(text)) if you want the nearest whole number instead. Other failing conversions: float("abc") raises ValueError, and int(None) raises TypeError. The ValueError page shows how to keep asking until the user types a valid number.