wow. what does that do ? I don't even know where to start looking that up.
I thought mandatory, optional, keyword args were enough...

Py3 feature: http://legacy.python.org/dev/peps/pep-3102/

This is the "bare star" argument passing style. It's not poorly implemented, it's just not implemented :) (On the compiler side it is, but not in the runtime side.)

Actually, while the compiler emits the correct byte code, it doesn't collect enough information about number of keyword-only args to pass to the runtime.

I'm working on this.

Well, in first approximation, kw-only args can be treated exactly as kw args. Of course, it would be nice if compiler set corresponding flag for a func arg. But handling it can be left for later, even #466 would be of higher priority.

It's not a flag, but rather the compiler needs to tell how many args are positional and how many are kw only. Then pretty much the existing code that handles kw args will work.

I again would like to sound a proposal that as first step, kw-only args should be treated just the same as normal args with a default value provided. In other words,

-    def __init__(self, _factory=message.Message, *, policy=compat32):

should be treated as:

+    def __init__(self, _factory=message.Message, policy=compat32):

Due to this simple handling not implemented, I have to apply patch like above manually to CPython stdlib.

I would rather fix it properly :)

Then friendly ping from urllib.request.urlopen():

def urlopen(url, data=None, timeout=socket._GLOBAL_DEFAULT_TIMEOUT,
            *, cafile=None, capath=None, cadefault=False):

We might now be able to close this issue :)

Great, thanks much! Seems to work based on one test ;-).