Throwing Your Own Errors

Throwing Your Own Errors

The welcome video’s JavaScript report rejected two rows with the reason ValueError is not defined. In this lesson, you found out why, by porting the line in Joan’s Python script that refuses a refund, like line 6 of the October export, -40.00.

For text like n/a, float() raises a ValueError on its own. A refund is a number, though, so parse_amount() raises its own ValueError, with a message that says what arrived. Joan calls it on line 6’s amount and logs the error’s type and its message:

def parse_amount(text):
    amount = float(text)
    if amount <= 0:
        raise ValueError(
            f"amount must be positive, got {amount}"
        )
    return amount

try:
    parse_amount("-40.00")
except ValueError as err:
    print(type(err).__name__, "-", err)

Python prints ValueError - amount must be positive, got -40.0.