At Dropbox I see a lot of people who want to start using type annotations (PEP 484, mypy) struggle with the collection ABCs. It's pretty common to want to support both sets and lists of values, as these have a lot of useful behavior: they are (re)iterable, have a size, and implement `__contains__` (i.e. x in c, x not in c). It's pretty common for people to think that Iterable is the answer, but it's not. I'm beginning to think that it would be useful to add another ABC to collections.abc (and to typing) that represents the intersection of these three ABCs, and to "upgrade" Set and Sequence to inherit from it. (And Mapping.) Thoughts? (Another useful concept is "reiterable", i.e. an Iterable that can be iterated over multiple times -- there's no ABC to indicate this concept. Sequence, Set and Mapping all support this concept, Iterator does not include it, but Iterable is wishy-washy.) -- --Guido van Rossum (python.org/~guido)