I almost posted this in “Ideas”, but I assume that there must be some design logic here that I have missed:
When an iterable ends, it throws a StopIteration exception. StopIteration exceptions hold a value (defaulting to None). This value can help indicate a final result before ending (such as using return inside a generator function). itertools.tee is designed to ““copy”” an iterator, however the iterables created by tee do not retain the StopIteration object thrown by the original iterator, nor even the value it held. Rather, it creates fresh StopIteration objects with value=None to throw instead.
Why? Wouldn’t it be better to save the original StopIteration object to throw for each child iterator? Or if mutability is a concern, then at least retain its value to construct the new StopIteration with? This issue also means that iterators that end with a subclass of StopIteration, if teed, will no longer produce this subclass anymore, potentially losing even more data.