Code to create a simple registerer.

create_registerer.py

"""This function returns two functions, the first is used to call by the registered
   functions based on the first argument(index or key). The second is a decorator
   used to register a new function. The decorator takes a key as argument or if no
   argument is provided, it taked the function name as key.
   
   Usage:
   call, register = create_registerer()
   
   @register('first')
   def a(arg): pass
   @register
   def b(arg): pass
   
   call('first', arg)  # a(arg)
   call(1, arg)  # a(arg)
   call('b', arg)  # b(arg)
   call(2, arg)  # b(arg)
   """

from functools import singledispatch
from itertools import islice


def create_registerer():
    @singledispatch
    def call(key, *args, **kwargs):
        """Use this function to call a registered function based on key"""
        return call.map[key](*args, **kwargs)

    @call.register
    def _(key: int, *args, **kwargs):
        """Use index rather than the key"""
        assert 1 <= key <= len(call.map)
        return next(islice(call.map.values(),
                           key - 1,
                           len(call.map)))(*args, **kwargs)

    call.map = {}

    def register_callee(key):
        """Use this decorator to register a callee"""
        assert not isinstance(key, int)

        def register(callee):
            call.map[key] = callee
            call.__doc__ += f'\n{len(call.map)}. {key}'
            return callee

        if callable(key):
            creator, key = key, key.__name__
            return register(creator)
        else:
            return register

    return call, register_callee