What is generator.throw() good for?

Question

PEP 342 (Coroutines via Enhanced Generators) added a throw() method to generator objects, which allows the caller to raise an exception inside the generator (as if it was thrown by the yield expression).

I am wondering what the use cases for this feature are.

Answer 1

Let's say I use a generator to handle adding information to a database; I use this to store network-received information, and by using a generator I can do this efficiently whenever I actually receive data, and do other things otherwise.

So, my generator first opens a database connection, and every time you send it something, it'll add a row:

def add_to_database(connection_string):
    db = mydatabaselibrary.connect(connection_string)
    cursor = db.cursor()
    while True:
        row = yield
        cursor.execute('INSERT INTO mytable VALUES(?, ?, ?)', row)

That is all fine and well; every time I .send() my data it'll insert a row.

But what if my database is transactional? How do I signal this generator when to commit the data to the database? And when to abort the transaction? Moreover, it is holding an open connection to the database, maybe I sometimes want it to close that connection to reclaim resources.

This is where the .throw() method comes in; with .throw() I can raise exceptions in that method to signal certain circumstances:

def add_to_database(connection_string):
    db = mydatabaselibrary.connect(connection_string)
    cursor = db.cursor()
    try:
        while True:
            try:
                row = yield
                cursor.execute('INSERT INTO mytable VALUES(?, ?, ?)', row)
            except CommitException:
                cursor.execute('COMMIT')
            except AbortException:
                cursor.execute('ABORT')
    finally:
        cursor.execute('ABORT')
        db.close()

The .close() method on a generator does essentially the same thing; it uses the GeneratorExit exception combined with .throw() to close a running generator.

All this is an important underpinning of how coroutines work; coroutines are essentially generators, together with some additional syntax to make writing a coroutine easier and clearer. But under the hood they are still built on the same yielding, and sending. And when you are running multiple coroutines in parallel, you need a way to cleanly exit those coroutines if one of them has failed, just to name an example.