The tee() function gives us a way to circumvent one of the important Python rules for working with iterables. The rule is so important, we'll repeat it here:
Iterators can be used only once.
The tee() function allows us to clone an iterator. This seems to free us from having to materialize a sequence so that we can make multiple passes over the data. For example, a simple average for an immense dataset could be written in the following way:
def mean(iterator: Iterator[float]) -> float: it0, it1 = tee(iterator,2)
N = sum(1 for x in it0)
s1 = sum(x for x in it1)
return s1/N
This would compute an average without appearing to materialize the entire dataset in memory in any form. Note that the type hint of float doesn't ...
