I found something else, I think it worth mentioning it.

This side-effect allows to create generators that return other values that None. And the CPython's behavior is not the same for all versions:

    >>> {(yield i) : i for i in range(2)}
    <generator object <dictcomp> at 0x7f0b98f41410>
    >>> list(_)
    # python3.(3,4,5)
    [0, 1]
    # python3.2
    [0, 1, {None: 1}] #  python3.2 appends the generator's return value.