What kind of objects `yield from` can be used with?

Question

Initially (PEP 380), yield from syntax was introduced to be used for delegating to a "subgenerator." Later it was used with now deprecated generator-based coroutines.

I cannot find out what kind of objects yield from can be applied to in general. My first conjecture was that it only requires __iter__ method on the object to return an iterator. Indeed, the following works with Python 3.8:

class C:
    def __init__(self, n):
        self.n = n

    def __iter__(self):
        return iter(range(self.n))

def g(n):
    yield from C(n)

print(tuple(g(3)))

However, it also works with some awaitables, like asyncio.sleep(1), which do not have __iter__ method.

What is the general rule? What determines if an object can be given as an argument to yield from form?

Answer 1

You can check how CPython evaluates that statement. From this follows it needs to be either a coroutine or an iterable:

case TARGET(GET_YIELD_FROM_ITER): {
    /* before: [obj]; after [getiter(obj)] */
    PyObject *iterable = TOP();
    PyObject *iter;
    if (PyCoro_CheckExact(iterable)) {
        /* `iterable` is a coroutine */
        if (!(co->co_flags & (CO_COROUTINE | CO_ITERABLE_COROUTINE))) {
            /* and it is used in a 'yield from' expression of a
               regular generator. */
            Py_DECREF(iterable);
            SET_TOP(NULL);
            _PyErr_SetString(tstate, PyExc_TypeError,
                             "cannot 'yield from' a coroutine object "
                             "in a non-coroutine generator");
            goto error;
        }
    }
    else if (!PyGen_CheckExact(iterable)) {
        /* `iterable` is not a generator. */
        iter = PyObject_GetIter(iterable);
        Py_DECREF(iterable);
        SET_TOP(iter);
        if (iter == NULL)
            goto error;
    }
    PREDICT(LOAD_CONST);
    DISPATCH();
}