[Python-ideas] channel (synchronous queue)

[Matt Joiner]

Recently (for some) the CSP style of channel has become quite popular
in concurrency implementations. This kind of channel allows sends that
do not complete until a receiver has actually taken the item. The
existing  queue.Queue would act like this if it didn't treat a queue
size of 0 as infinite capacity.

In particular, I find channels to have value when sending data between
threads, where it doesn't make sense to proceed until some current
item has been accepted. This is useful when items are not purely CPU
bound, and so generators are not appropriate.

I believe this rendezvous behaviour can be added to queue.Queue for
the maxsize=0 case, with maxsize=None being the existing "infinite
queue" behaviour. Additionally a close method, Closed exception and
other usability features like an __iter__ for receiving until closed
can be added. The stackless class linked below also has some other
possible ideas for performance reasons that make a lot of sense.

Existing code using queue.Queue would remain completely unaffected by
such additions if the default maxsize value is changed to
maxsize=None, and maxsize=0 is not being explicitly passed (it's
currently the default).

Here are a few links for some background and ideas:
http://gevent.org/gevent.queue.html#gevent.queue.Queue
http://www.disinterest.org/resource/stackless/2.6-docs-html/library/stackless/channels.html#the-channel-class
http://en.wikipedia.org/wiki/Communicating_sequential_processes#Comparison_with_the_Actor_Model
http://golang.org/doc/go_spec.html#Channel_types