[Python-ideas] Suggested MapView object (Re: __len__() for map())

[E. Madison Bray]

On Sun, Dec 2, 2018 at 11:52 PM Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>
> Steven D'Aprano wrote:
> > Perhaps more like the principle of most
> > astonishment: the object changes from sized to unsized even if you don't
> > modify its value or its type, but merely if you look at it the wrong
> > way:
>
> Yes, but keep in mind the purpose of the whole thing is to
> provide a sequence interface while not breaking old code
> that expects an iterator interface. Code that was written
> to work with the existing map() will not be calling len()
> on it at all, because that would never have worked.
>
> > Neither fish nor fowl with a confusing API that is not
> > quite a sequence, not quite an iterator, not quite sized, but just
> > enough of each to lead people into error.
>
> Yes, it's a compromise in the interests of backwards
> compatibility. But there are no surprises as long as you
> stick to one interface or the other. Weird things happen
> if you mix them up, but sane code won't be doing that.

Indeed; I believe it is very useful to have a map-like object that is
effectively an augmented list/sequence.