Wednesday, May 24, 2006

DeferArgs lets you write syncronous looking functions that really aren't!

So a little side project for my own uses provided a simple decorator that lets me write a function I can pass deferreds and regular arguments to, and have the function return a deferred that fires when all of its deferred arguments are ready and the function has processed them. Some example usage:


@deferargs
def printArgs(*args, **kwargs):
print "Positional Arguments: ", ", ".join(args)
print "Keyword Arguments: ", ", ".join("%r=%s"%(k,v) for (k,v) in kwargs.items())

printArgs("foobar", baz=someNetworkRequest())


Really basic, but it can prove useful for a large portion of Twisted code you might write. I'm planning to add some semi-evil way to do something that looks a lot like a try/except/finally block but is actually (obviously) not, and works with any errbacks from deferreds within the try-like block of code. The reaction has been interesting. I've had some people stand up for the idea, which is similar to
defgen
, and others who think it is a bad idiom that is dangerous to encourage.

Using this kind of abstraction over asyncronous code, you do have to be careful to remember what is asyncronous and take consideration that your code won't run until all deferred arguments are ready, even if some parts could be run with only some of the arguments. In those cases, however, you should just break up the function, and I'd like to note that you can make the same mistakes using deferreds and the like directly, so I don't really see it as an issue.

If you want to do any error handling for the moment, you need to attach errbacks to the deferred from the function call. I want to work in my semi-evil error handling soon, because my goal here is to hide the fact that there are deferreds as much as possible, but for now this is just fine, and I've already had use of it myself.

You can get it from the Cheeseshop, so check it out there now and place any comments about it here.

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2 comments:

Kevin Deenanauth said...

Thanks for this! I think this goes a long way toward making Twisted code more readable (and Pythonic =)

The concerns for this is understandable, but if you know how Twisted works, then there shouldn't be a problem.

Anonymous said...

This is very cool. I don't think it is counter to the way twisted processing flow really works at all.

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