PEP 563 -- Postponed Evaluation of Annotations (https://www.python.org/dev/peps/pep-0563/)

defaults.py

from __future__ import annotations
from inspect import signature
from functools import wraps
from collections import namedtuple

opt = namedtuple('Optional', '')()
def better_defaults(f):
    sig = signature(f)
    @wraps(f)
    def func(*args, **kwargs):
        ba = sig.bind(*args, **kwargs)
        ba.apply_defaults()
        kwargs.update({name: eval(f.__annotations__[name], ba.arguments.copy())
                       for name, val in ba.arguments.items() if val is opt})
        return f(*args, **kwargs)
    return func

@better_defaults
def f(x: y * 10  = opt,
      y: x * 100 = opt,
      z = 10):
    return x + y + z

print(f'f(x=1) {f(1)}')
print(f'f(x=1) {f(x=1)}')
print(f'f(y=2) {f(y=2)}')
print(f'f(x=1) {f(1, z=3)}')
print(f'f(x=1) {f(1, y=2, z=3)}')