Python Casting

Converting types

Use the type's constructor to convert a value:

print(int("42") + 1)
print(float("3.5"))
print(str(10) + " apples")
print(int(3.99))
print(round(3.99))

43
3.5
10 apples
3
4

Common gotchas

• int("3.9") fails — convert to float first, then to int.
• int() truncates towards zero; use round() to round.
• Text from users, files and APIs is always a str — convert before doing math.

price = "19.99"
quantity = "3"
total = float(price) * int(quantity)
print(total)
print(round(total, 2))

59.97
59.97

Handling invalid input

Converting invalid text raises a ValueError. Catch it with try/except (covered later):

try:
    age = int("twenty")
except ValueError as error:
    print("Not a number:", error)

Not a number: invalid literal for int() with base 10: 'twenty'