Raising and custom exceptions

Refusing to continue

Sometimes the right answer is to fail. raise does that:

def withdraw(balance, amount):
    if amount > balance:
        raise ValueError("insufficient funds")
    return balance - amount

Returning None or -1 for an impossible request only postpones the problem: the caller forgets to check, and the bad value spreads. An exception cannot be ignored by accident.

Define your own when callers need to catch your failure specifically:

class InsufficientFunds(Exception):
    pass

Inheriting from Exception is all that is required. Now a caller can write except InsufficientFunds: and leave everything else alone.

Your turn: define InsufficientFunds and write withdraw(balance, amount) that raises it when the amount is more than the balance, and ValueError when the amount is zero or negative.