Inner decorator

does_work.py

import types
import opcode
MAKE_FUNCTION = opcode.opmap['MAKE_FUNCTION']
del opcode

def decorate_inner(decorator, name):
    def actual_decorator(outer_fn):
        # Try and find the decorator function in co_consts,
        # perform EAFP
        co_consts = outer_fn.__code__.co_consts
        try:
            index = co_consts.index(decorator)
        except ValueError:
            index = len(co_consts)
            outer_fn.__code__ = outer_fn.__code__.replace(
                co_consts=co_consts + (decorator,)
            )

        # Try and find the name in co_consts,
        # perform EAFP
        try:
            index2 = co_consts.index(
                f"{outer_fn.__code__.co_name}.<locals>.{name}"
            )
        except ValueError:
            raise RuntimeError(f"couldn't find name {name}") from None

        if index2 > 127:
            raise IndexError(f"name index in co_consts is > 127")

        co_code = outer_fn.__code__.co_code
        character = chr(index2)
        base = co_code.find(f'd{character}'.encode())

        while co_code[base+2] is not MAKE_FUNCTION:
            base = co_code.find(f'd{character}'.encode(), base+2)

        outer_fn.__code__ = outer_fn.__code__.replace(
            co_code=(co_code[:base+4]
                    + f'd{chr(index)}\x02\x00\x83\x01'.encode('l1')
                    + co_code[base+4:])
        )
        return outer_fn
    return actual_decorator


def deco(fn):
    def wrapper(*args, **kwargs):
        print("Calling", fn.__name__)
        ret = fn()
        print("Called", fn.__name__)
        return ret

    return wrapper


@decorate_inner(deco, "g")
def f():
    def g():
        return 42
    return g


g = f()
print(g())