accept loop= kwarg

[meejah]

See https://github.com/crossbario/autobahn-python/issues/747 for context.

asyncio allows passing in a loop= kwarg to the Future constructor. Currently, we always pass the One True Loop (whatever config.loop says).

txaio.create_future APIs should accept an (optional) loop= kwarg (or reactor= or loop_or_reactor=?). It's an error on Twisted if this is not the same object as config.loop. For asyncio, we pass it along to Future.

The affected APIs are:

• txaio.create_future
• txaio.as_future
• txaio.call_later
• txaio.make_batched_timer

[meejah]

The tough ones are as_future and call_later as they don't have a nice way to just add a new loop=None kwarg.

We discussed a few options on IRC, and concluded:

• breaking the API totally by adding a positional "loop" arg is Very Bad
• pop()-ing "loop" from kwargs is Bad (and will lead to weird, surprising errors in code that happens to already take a loop kwarg)
• using something like the testutils.replace_loop context manager is weird and might not work reliably anyway

So we will try by introducing an "instantiate an API object"-type method (final naming TBD) that has all the txaio API methods hanging off it and takes an (optional) custom loop argument. So code like this:

f0 = txaio.as_future(...)
f1 = txaio.call_later(...)

becomes:

api = txaio.create_api(loop=asyncio.new_event_loop())
f0 = api.as_future(...)
f1 = api.call_later(...)

This won't change any existing code, but code that wants to use many custom loops can opt-in by using the .create_api() call.

[meejah]

Worth noting: you can already use an explicit event-loop (or reactor), but only one. This feature is for users of asyncio to use multiple event loops at once. Twisted doesn't (yet) support this.

[meejah]

See https://github.com/crossbario/txaio/pull/98#issuecomment-292713470