Which part of the behaviour do you think is a bug?

In version1, you want to think of the yield expression as though it were bracketed like this:

    result = yield (a+100 if b>100 else (yield a))

With the particular values of a and b that you have, that statement is equivalent to this:

    result = yield (yield 0)

This yields two values to the consumer: a `0` from the inner yield, followed by a None (or whatever was "sent" into the generator, if applicable) for the out yield.

In version 2, the corresponding line simplifies to just

    result = yield 0

So no, I don't think these two versions of the code should behave the same, and I don't think there's any bug here.