Challenge 3: Custom Exception for Stock Levels — Possible Solution
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class OutOfStockError(Exception):
    pass

def purchase(stock, quantity):
    if quantity > stock:
        raise OutOfStockError(f"Cannot buy {quantity}, only {stock} in stock")
    return stock - quantity

try:
    remaining = purchase(10, 3)
    print(f"Purchase succeeded, {remaining} remaining")
except OutOfStockError as e:
    print(f"Error: {e}")

try:
    purchase(10, 15)
except OutOfStockError as e:
    print(f"Error: {e}")

Output:
Purchase succeeded, 7 remaining
Error: Cannot buy 15, only 10 in stock

WHY THIS WORKS AS AN ANSWER
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class OutOfStockError(Exception): pass reuses the chapter's own
InsufficientFundsError example precisely — a custom exception is just
a class inheriting from Exception, with pass as the entire body being
enough to make it a real, distinct, catchable exception type.

purchase(stock, quantity) mirrors the chapter's withdraw() function
structure: raise OutOfStockError(f"...") fires only when the
requested quantity exceeds available stock, carrying a message built
from an f-string that includes both numbers involved — the same
"specific, descriptive message" approach the chapter's own custom
exception example demonstrated.

The first purchase(10, 3) call succeeds normally (3 does not exceed
10), so purchase returns stock - quantity = 7, and the try block's
print statement runs. The second purchase(10, 15) call does exceed
available stock, so it raises OutOfStockError, which is caught
specifically by except OutOfStockError as e: — catching this custom
type by name, rather than a generic Exception, makes it immediately
clear from the except clause itself what failure is being handled.