Single-dispatch generic function decorator.

    Transforms a function into a generic function, which can have different
    behaviours depending upon the type of its first argument. The decorated
    function acts as the default implementation, and additional
    implementations can be registered using the register() attribute of the
    generic function.

def singledispatch(function):  # noqa
    """Single-dispatch generic function decorator.

    Transforms a function into a generic function, which can have different
    behaviours depending upon the type of its first argument. The decorated
    function acts as the default implementation, and additional
    implementations can be registered using the register() attribute of the
    generic function.

    """
    registry = {}
    dispatch_cache = WeakKeyDictionary()

    def ns():
        pass

    ns.cache_token = None

    # noinspection PyIncorrectDocstring
    def dispatch(cls):
        """generic_func.dispatch(cls) -> <function implementation>

        Runs the dispatch algorithm to return the best available implementation
        for the given *cls* registered on *generic_func*.

        """
        if ns.cache_token is not None:
            current_token = get_cache_token()
            if ns.cache_token != current_token:
                dispatch_cache.clear()
                ns.cache_token = current_token
        try:
            impl = dispatch_cache[cls]
        except KeyError:
            try:
                impl = registry[cls]
            except KeyError:
                impl = _find_impl(cls, registry)
            dispatch_cache[cls] = impl
        return impl

    # noinspection PyIncorrectDocstring
    def register(cls, func=None):
        """generic_func.register(cls, func) -> func

        Registers a new implementation for the given *cls* on a *generic_func*.

        """
        if func is None:
            return lambda f: register(cls, f)
        registry[cls] = func
        if ns.cache_token is None and hasattr(cls, '__abstractmethods__'):
            ns.cache_token = get_cache_token()
        dispatch_cache.clear()
        return func

    def wrapper(*args, **kw):
        return dispatch(args[0].__class__)(*args, **kw)

    registry[object] = function
    wrapper.register = register
    wrapper.dispatch = dispatch
    wrapper.registry = MappingProxyType(registry)
    wrapper._clear_cache = dispatch_cache.clear
    update_wrapper(wrapper, function)
    return wrapper