Raising custom exceptions in Python

Naming your failure modes with your own exception class.

class EmptyCartError(Exception):
    pass


def checkout(cart):
    if not cart:
        raise EmptyCartError("nothing to buy")
    if any(price < 0 for price in cart.values()):
        raise ValueError("negative price")
    return sum(cart.values())


try:
    total = checkout({})
except EmptyCartError as exc:
    total = 0

How it works

1. EmptyCartError subclasses Exception in one line.
2. raise aborts checkout with a specific, catchable type.
3. The caller handles it separately from generic errors.

Keywords and builtins used here

• EmptyCartError
• any
• as
• checkout
• class
• def
• except
• for
• if
• pass
• raise
• return
• sum
• try

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Step 5 of 7 in Iterators, errors & files, step 48 of 72 in Language basics.

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