Custom exception with attributes

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Accepted Answer

The custom exception stores amount=100 and balance=40 as instance attributes, so e.amount - e.balance is 60. Calling str(e) (or printing e directly) returns the message passed to super().__init__, which is "need 100, have 40". This pattern - storing structured data as attributes alongside a human-readable message - is exactly why you define custom exceptions: callers can programmatically inspect e.amount and e.balance rather than parsing a string.