This library has been factored out of the Autobahn|Python WAMP client
ApplicationSession object from that project therefore
serves as a good example of how to use this library in a complex
We are releasing it in the hopes these utilities are useful on their own to other projects using event-based Python. Only authors of “library style” code are likely to be interested in this – new application code should use your favourite Python asynchronous I/O platform.
Overview by Example¶
The simplest way to use txaio is to
import txaio and use the
helper functions directly. You must select the framework you wish to
use by calling
(which means asyncio, or trollius/tuplip if asyncio import fails).
Note that to use this library successfully you shouldn’t call methods on futures – use only txaio methods to operate on them.
import txaio txaio.use_twisted() # or .use_asyncio() def cb(value): print("Callback:", value) def eb(fail): # fail will implement txaio.IFailedFuture print("Errback:", txaio.failure_message(fail)) print(txaio.failure_formatted_traceback(fail)) f = txaio.create_future() txaio.add_callbacks(f, cb, eb) # ...other things happen... try: answer = do_something() fail = None except Exception: fail = txaio.create_failure() # the point here is that you "somehow" arrange to call either # reject() or resolve() on every future you've created. if fail: txaio.reject(f, fail) else: txaio.resolve(f, answer)
Restrictions and Caveats¶
txaio is not a new event-based programming solution. It is not a complete box-set of asynchronous programming tools.
It is one piece that can help library authors to write cross-event-loop asynchronous code. For example, you’ll note that there’s no way to run “the event loop” – that’s up to you.
There is no support for @coroutine or @inlineCallbacks
decorators. This is not possible, as asyncio under Python3 introduced
a new syntax (
yield from) to call into other co-routines. So, you
are stuck with “callback style” code for your cross-platform
library. (Note that users of your library can of course use new
Python3 features like
await in their
own code – but they do so by explicitly choosing “Python3 and
asyncio” as their platform).
txaio is basically a “lowest common denominator” tool. There is a
minimum of wrapping, etcetera but the library author doesn’t get to
use fancy features (e.g.
@inlineCallbacks, mutation of returns,
@coroutine) of the underlying async platforms.
Futures and Deferreds¶
In most cases asyncio is trying to be “as thin as possible” wrapper
around the different APIs. So, there’s nothing wrapping Future or
Deferred – you get the bare objects. This means that
txaio.create_future() returns you the native object, which
you then pass to
txaio.call_later() returns the underlying object
IDelayedCall in Twisted or a
Handle in asyncio). These both
cancel() method, but little else in common.
Callbacks and Errbacks¶
Twisted and asyncio have made different design-decisions. One that
stands out is callbacks, and callback chaining. In Twisted, the return
value from an earlier callback is what gets passed to the next
callback. Similarly, errbacks in Twisted can cancel the error. There
are not equivalent facilities in
asyncio: if you add multiple
callbacks, they all get the same value (or exception).
When using txaio, don’t depend on chaining. This means that
errback methods must always return their
input argument so that Twisted works if you add multiple callbacks
or errbacks (and doesn’t unexpectedly cancel errors).
txaio does add the concept of an
errback for handling errors
(a concept asyncio does not have) and therefore adds one helper to
encapsulate exceptions (similar to Twisted’s Failure object) which
only exists in the asyncio implementation.
There is no
support. Don’t use these.
errback, you will receive a single arg which is an
instance conforming to
IFailedFuture. This interface has only a
.value, which is the Exception instance which
caused the error. You can also use
txaio.failure_* methods to
operate on an
- txaio.failure_message: returns a unicode error-message
- txaio.failure_traceback: returns a
- txaio.failure_formatted_traceback: returns a unicode formatted stack-trace
You should not depend on any other attributes or methods of the instance you’re given.
You are encouraged to look at Autobahn|Python for an example of a
system that can run on both Twisted and asyncio. In particular, look
at the difference between
autobahn/asyncio/websocket.py and the compatibility super-class in
autobahn/wamp/protocol.py which is the piece that uses txaio
to provide an event-loop agnostic implementation that both the Twisted
and asyncio concrete
ApplicationSession objects inherit from.
autobahn.wamp.protocol.ApplicationSession is glued to a particular
takes advantage of
txaio.tx.LoopMixin to provide the
helpers-methods attached to
In this manner, code in the generic implementation simply always calls
txaio methods via
self.create_future() or similar and users of
Autobahn|Python can choose between asyncio and Twisted as they prefer
from autobahn.twisted.wamp import ApplicationSession or
from autobahn.asyncio.wamp import ApplicationSession