2019 08 Python: Fix problem with islicing map
2019 08 Python: Fix problem with islicing map

Last three rows were coming up empty, which I'm fairly sure was a result of islicing the single-pass iterator from map.

There's no real gain here from operating on the iterator, so instead I'm now just creating a list, which makes the matter much more straightforward to reason about.

Note that I'm still using `iter(self)`  rather than just `self` to be more explicit about what's going on with the zip (zipping together the layers).

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