Programming Fundamentals › Error Handling

Throwing / Raising

Signalling an error explicitly.

Also known as: raise, throwing exceptions, raising exceptions

Throwing (or raising) an exception is how code signals that something is wrong. Execution stops at that line, and the exception travels up to the nearest handler.

def withdraw(balance, amount):
    if amount <= 0:
        raise ValueError("amount must be positive")
    if amount > balance:
        raise InsufficientFunds(balance, amount)
    return balance - amount

function withdraw(balance, amount) {
  if (amount <= 0) throw new Error("amount must be positive");
  return balance - amount;
}

Python and Ruby use raise. Java, JavaScript, C# and C++ use throw.

When to throw

• A function can’t do its job because inputs or the state are invalid.
• A needed resource isn’t available.
• A rule has been broken, and returning a “fake” value would hide the problem.

Check at the top and throw early (guard clauses). Don’t return None or -1 for “error” unless callers are meant to check.

Good habits

• Throw specific types with clear messages: what was wrong, with which value (custom exceptions).
• Throw objects, not strings (in JavaScript, throw new Error(...), not throw "oops", which has no stack trace).
• Don’t use exceptions for ordinary control flow, like ending a loop.
• Preserve the cause when you rethrow a different exception (raise X from e).
• Don’t throw from places that can’t handle it, like destructors or cleanup code.
• Document what a function can throw.

Someone has to catch it, or the program stops (try / catch).