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Iterators, iterables and special objects

On Tue, Jul 21, 2020, at 15:54, Terry Reedy wrote:
> The transformers should be once-through iterators because they can be 
> passed once-through interators.  I suppose one could make them iterables 
> and add an attribute 'pristine' set to True in __init__ and False in 
> __iter__, but why have 2 objects instead of 1 when there is not gain in 
> function?

Why not just allow them to be iterated multiple times, and the underlying iterator/iterable either handles that or doesn't as the case may be? We don't have a hard API distinction between iterables and iterators, all iterators are "iterable" in the sense that they have their own __iter__ method that returns self.

i.e. the equivalent of

class map:
    def __init__(self, func, obj): ...
    def __iter__(self): for x in iter(self.obj): yield self.func(x)

That way if it is passed a once-through iterator, it is a once-through iterator with a couple extra steps, if passed an iterable it's an iterable.