Programs that talk to the user

input() reads one line from the user and always gives a string — even if the user types digits.

age = input("How old are you? ")
print(age + 1)        # TypeError! age is "12", not 12

You have to convert: int(age). But if the user types «twelve», int() crashes. Real programs have to survive that.

def read_number(question, attempts=3):
    for _ in range(attempts):
        answer = input(question).strip().replace(",", ".")
        try:
            return float(answer)
        except ValueError:
            print(f"'{answer}' is not a number. Try again.")
    raise ValueError("too many invalid attempts")

age = read_number("How old are you? ")
print(f"Next year you turn {age + 1:.0f}")

Three things that make this function good:

• .strip() removes spaces the user typed by accident.
• .replace(",", ".") accepts the Swedish decimal comma.
• try/except catches the error and asks again instead of crashing.

Always test your program with: an ordinary value, an empty answer, text instead of a number, and a negative number. Those four reveal the bugs.