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How does a try/except block work, and what happens if no matching except clause exists? What does the 'finally' block guarantee, and when is it typically used? How do you catch multiple exception types with a single except clause? Why is a bare 'except:' clause (catching everything with no type specified) generally discouraged? What is the difference between 'raise' and 'raise ... from ...' for exception chaining? How do you access details about the exception currently being handled, like its type, value, and traceback, using sys.exc_info()? How does the 'else' clause on a try statement work, and what's its purpose compared to just putting that code at the end of 'try'? How does Python 3.11's exception groups (ExceptionGroup) and the 'except*' syntax let you handle MULTIPLE unrelated exceptions raised together? How would you design a custom exception HIERARCHY for an application, and why does this matter for how callers can catch errors at different granularities? How does context manager __exit__ interact with exception handling, and how can it SUPPRESS an exception by returning True?
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Python › Exception Handling

How do you create a custom exception class, and what's the minimal amount of code needed?

Intermediate

Answer

Define a class inheriting from Exception (or a more specific built-in exception), typically with no additional code needed at all (just 'pass') unless you want to add custom attributes or override __init__ — Python's exception machinery handles the rest automatically via inheritance.
class InsufficientFundsError(Exception):
    def __init__(self, shortfall):
        super().__init__(f"Insufficient funds. Short by ${shortfall:.2f}")
        self.shortfall = shortfall

try:
    raise InsufficientFundsError(50.00)
except InsufficientFundsError as e:
    print(e.shortfall)  # 50.0
Real-world example Raising a domain-specific exception (like InsufficientFundsError) that carries meaningful business context beyond a generic message.

Common follow-ups: Why is it good practice to inherit custom exceptions from a specific built-in type rather than always using the bare 'Exception'?

OOP
All 15 Exception Handling questions