I believe Yury already figured out what I meant, but to make it entirely clear, after the change, this example:

   def foo(a, b): pass
   foo_partial = functools.partial(foo, 'spam')
   foo_partial2 = functools.partial(foo, a='spam')

Should lead to the following signature for both "foo_partial" and "foo_partial2":

    (b, *, a='spam')

That accurately indicates that "a" is now effectively a keyword only parameter - the first supplied positional argument will map to "b", and attempting to supply *two* positional arguments will fail.

Correctly handing *args may get a little interesting, but should be feasible.