If you accidentally iterate over aUnion, you get a hang:

fromtypingimportUnionlist(Union)# ...hang

It looks like it's an infinite loop:

fromtypingimportUnionforiinUnion:print(i)# 0, 1, 2, ...

As far as I can tell, this happens becausetyping.Union is implemented usingtyping._SpecialForm, and_SpecialForm is implemented using__getitem__ to support parameterisation likeUnion[int]. Unfortunately, because_SpecialForm doesn't implement__iter__,__getitem__ is also used for iteration - meaning that when trying to iterate, we effectively doUnion[0],Union[1], etc.