Custom Exception Inventory

Built-in exceptions don't always describe your problem precisely. You can define your own by subclassing Exception.

Define an exception class InsufficientStockError (subclassing Exception), then write:

remove_stock(stock, amount)

If amount is greater than stock, raise InsufficientStockError. Otherwise, return stock - amount.

Examples

remove_stock(10, 4)    // → 6
remove_stock(10, 10)   // → 0
remove_stock(10, 15)   // raises InsufficientStockError

Walkthrough

Thinking it through

Built-in exceptions like ValueError are generic on purpose — they describe a shape of problem ("bad value"), not your specific domain. When you're modeling something like an inventory system, a custom exception class communicates intent far better than reusing ValueError for everything.

Defining a custom exception

class InsufficientStockError(Exception):
    pass

That's the entire definition. Subclassing Exception gets you everything a normal exception needs — a message, a traceback, the ability to be caught by type — for free. You don't need to override __init__ unless you want extra structured data on the exception itself (some problems later in this course will).

Using it

Once defined, you raise and catch it exactly like a built-in:

if amount > stock:
    raise InsufficientStockError(f"only {stock} in stock, tried to remove {amount}")

Why this matters

As programs grow, catching except InsufficientStockError: specifically (rather than a generic except ValueError: or, worse, except Exception:) means calling code can respond precisely to this failure mode without accidentally swallowing unrelated bugs that also happen to raise ValueError.

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