I had a quick discussion with @NicolasHug following the discussion of keyword only arguments on the mailing list.

That should be possible to deprecate the use of positional arguments via a decorator, so that

def myfunction(a=1, b=2, c=3):
    return a + b + c

will become

def myfunction(*, a=1, b=2, c=3):
    return a + b + c

but will not break, but raise a deprecation warning if used with positional arguments.

Might look like:

from warnings import warn
from functools import wraps

def warn_args_wrapper(f):
    @wraps(f)
    def new_f(*args, **kwargs):
        if args:
            warn(DeprecationWarning("got args {}, you should use keyword args!".format(args)))
        return f(*args, **kwargs)
    return new_f

@warn_args_wrapper
def f(a=1, b=2, c=3):
    return a + b + c

We would need to put that decorator around all __init__ definitions (at least).
It's a big magic and I'm not sure how well it plays with sphinx.

This RFC is to float the idea and discuss whether it's worth writing a SLEP / doing a prototype.