Here's a proof-of-concept for the decorator. It does not address the issue of passing aliases to positional arguments to **kwargs - I guess this requires changes in the CPython's core.

(Sorry about the coloring, that's how it's pasted)

from inspect import signature, Parameter
from functools import wraps


def positional_only(n):
    def wrap(f):
        s = signature(f)
        params = list(s.parameters.values())
        for i in range(n):
            if params[i].kind != Parameter.POSITIONAL_OR_KEYWORD:
                raise TypeError('{} has less than {} positional arguments'.format(f.__name__, n))
            params[i] = params[i].replace(kind=Parameter.POSITIONAL_ONLY)
        f.__signature__ = s.replace(parameters=params)

        @wraps(f)
        def inner(*args, **kwargs):
            if len(args) < n:
                raise TypeError('{} takes at least {} positional arguments'.format(f.__name__, n))
            return f(*args, **kwargs)
        return inner
    return wrap


@positional_only(2)
def f(a, b, c):
    print(a, b, c)


help(f)
# f(a, b, /, c, **kwargs)

f(1, 2, c=2)

# f(1, b=2, c=3)
# TypeError: f takes at least 2 positional arguments


@positional_only(3)
def g(a, b, *, c):
    print(a, b, c)

# TypeError: g has less than 3 positional arguments

Elazar

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