Message 223026 - Python tracker
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.