[Python-ideas] async: feedback on EventLoop API

[Shane Green]

Ignoring the API overlap with Futures/Promises for a moment, let me throw out this straw man approach to the event loop that seems to my naive eye like it pull together a lot of these ideas…

Rather than passing in your callbacks, factories, etc., asynchronous APIs return a lightweight object you register your callback with.  Unlike promises, deferrers, etc., this is a one-time thing: only one callback can register with it.  However, it can be chained.  The registered callback is invoked with the output of the operation when it completes.  

Timer.wait(20).then(callme, *args, **kw)
# I could do 
Timer.wait(20).then(callme, *args, **kw).then(piped_from_callme) 

#I could not do 
handler = Timer.wait(20)
handler.then(callme)
handler.then(callme2) # this would throw an exception.

# I/O example…
sock.accept().then(handle_connection) # invokes handle_connection(conn, addr)
# Read some data
conn.read(1024).then(handle_incoming) # handle_incoming invoked with up to 1024 bytes, read asynchronously.
# Write some data
conn.write("data").then(handle_written) # handle_written invoked with up number 5, giving number of bytes written async. 
# Connect HTTP channel and add it to HTTP dispatcher.
channel.connect((hostname,80)).then(dispatcher.add_channel)


# Listen to FD's for I/O events
descriptors.select(r, w, e).then(handle) # handle(readable, writables, oobs) 

It seems like only supporting a single callback per returned handle lets us circumvent a lot of the weight associated with normal promise/future/deferred pattern type implementations, but the chaining could come in handy as it may cover some of the use-cases being considered when multiple events per fd came up, plus chaining is pretty powerful, especially when it comes at little cost.  The API would be much more extensive than "then()", of course, with things like "every", etc. we'd have to pull examples from everything already discussed.  Just wanted to throw out there to get beat up about ;-) 



Shane Green 
www.umbrellacode.com
805-452-9666 | shane at umbrellacode.com

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