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Python / Core Python Fundamentals Interview Questions

What is the Python exception class hierarchy and how do you create custom exceptions?

Python exceptions form a class hierarchy rooted at BaseException. Most exceptions you deal with inherit from Exception, which itself inherits from BaseException. The hierarchy determines which except clauses match a raised exception — a handler for a parent class catches instances of all child classes.

# BaseException
#   ├── SystemExit          # sys.exit()
#   ├── KeyboardInterrupt   # Ctrl-C
#   ├── GeneratorExit       # generator.close()
#   └── Exception           # all regular exceptions
#         ├── ValueError
#         ├── TypeError
#         ├── AttributeError
#         ├── KeyError
#         ├── IndexError
#         ├── RuntimeError
#         │     └── RecursionError
#         ├── OSError
#         │     ├── FileNotFoundError
#         │     └── PermissionError
#         └── ArithmeticError
#               └── ZeroDivisionError

Creating custom exceptions is simple — subclass Exception (or a more specific built-in) and optionally add an __init__ for structured error data:

class InsufficientFundsError(ValueError):
    def __init__(self, balance, amount):
        self.balance = balance
        self.amount  = amount
        super().__init__(
            f'Cannot withdraw {amount}; balance is only {balance}')

def withdraw(account, amount):
    if amount > account.balance:
        raise InsufficientFundsError(account.balance, amount)
    account.balance -= amount

try:
    withdraw(acc, 9999)
except InsufficientFundsError as e:
    print(e)            # Cannot withdraw 9999; balance is only 1500
    print(e.amount)     # 9999 — structured access

Which exception class should you subclass for most custom business logic exceptions?
Catching 'except OSError' will also catch which of the following?

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