Reference the value of other arguments in your function definitions

args.py

import functools
import inspect
import itertools
import typing


class Arg(typing.NamedTuple):
    name: str


def wrap(func):
    sig = inspect.signature(func)

    # ensure no Args point to another Arg
    # eg no `def foo(a, b=Arg('a'), c=Arg('b')):`

    for val in sig.parameters.values():
        if isinstance(val.default, Arg):
            arg: str = val.default.name
            try:
                pointer: inspect.Parameter = sig.parameters[arg]
            except KeyError:
                raise TypeError(
                    "Arg '{}' points to nonexistent argument '{}'".format(
                        val.name, val.default.name
                    )
                )

            if isinstance(pointer.default, Arg):
                raise TypeError(
                    "Arg '{}' points to another Arg '{}' = Arg('{}')".format(
                        val.name, pointer.name, pointer.default.name
                    )
                )

    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        # ensure no Args passed in at runtime
        # eg no `foo(Arg('b'))`

        for val in itertools.chain(args, kwargs.values()):
            if isinstance(val, Arg):
                raise TypeError(f"Cannot pass in Arg instance {val}")

        # Will raise error on incorrect *args, **kwargs
        bound = sig.bind(*args, **kwargs)
        bound.apply_defaults()

        # Loop through all arguments, replacing Args with the correct value
        for key, val in bound.arguments.items():
            if isinstance(val, Arg):
                bound.arguments[key] = bound.arguments[val.name]

        return func(*bound.args, **bound.kwargs)

    return wrapper


@wrap
def foo(a, b, c=None, d=Arg("c"), e=Arg("b")):
    return {"a": a, "b": b, "c": c, "d": d, "e": e}


def test(*args, **kargs):
    print(f"Result of calling foo({args} {kargs}):")
    print(foo(*args, **kargs))
    print()


test(1, 2)
test(5, 7, d=0)