It is at least an omission from the documentation. The glossary <https://docs.python.org/dev/glossary.html#term-mapping> refers to the Mapping ABC. From Christian’s point of view, the quack of an iterator with just a “keys” attribute sounds more like an iterator than a mapping.

I think the documentation for the dict() constructor should say how to ensure the iterable and mapping modes are triggered. Perhaps dict.update() should also, because it appears to also treat non-dict() “mappings” differently to plain iterators.