On 27 Apr 2015 07:50, "Mark Shannon" <m...@hotpy.org> wrote: > On 26/04/15 21:40, Yury Selivanov wrote: >> >> But it's hard. Iterating through something asynchronously? Write a >> 'while True' loop. Instead of 1 line you now have 5 or 6. Want to >> commit your database transaction? Instead of 'async with' you will >> write 'try..except..finally' block, with a very high probability to >> introduce a bug, because you don't rollback or commit properly or >> propagate exception. > > I don't see why you can't do transactions using a 'with' statement.
Because you need to pass control back to the event loop from the *__exit__* method in order to wait for the commit/rollback operation without blocking the scheduler. The "with (yield from cm())" formulation doesn't allow either __enter__ *or* __exit__ to suspend the coroutine to wait for IO, so you have to do the IO up front and return a fully synchronous (but still non-blocking) CM as the result. We knew about these problems going into PEP 3156 ( http://python-notes.curiousefficiency.org/en/latest/pep_ideas/async_programming.html#using-special-methods-in-explicitly-asynchronous-code) so it's mainly a matter of having enough experience with asyncio now to be able to suggest specific syntactic sugar to make the right way and the easy way the same way. Cheers, Nick.
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