What is `yield from`?

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Exactly what it sounds like: it yields the values from the other source, one at a time, until it runs out. Every time there is a request for the next element from this generator, if there is still something in the other source, it will use that. Otherwise, the logic proceeds.

The loop

while True:
    print('I am going to yield something!')
    yield from normal_number_generator

is effectively equivalent to:

while True:
    print('I am going to yield something!')
    for i in normal_number_generator:
        yield i

Because yield from makes all of the values from normal_number_generator get yielded (and thus printed in the main loop), but then the while True: loop does not resume, because there are no more next calls on that generator - so it’s “stuck” on the yield from line. If you try next(zero_to_ten) again after the code you showed, it will get in an infinite loop, because the while True: loop can continue, but yield from doesn’t have any more elements to offer, so it tries forever looking for the next actual value to yield.

Why would it? It does its own iteration for “that iterable”, which has to finish before your while True: loop can proceed.

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