INADA Naoki added the comment: So, I feel the 2nd meaning of "sequence" should be collections.abc.(Mutable)Sequence. "sequence types in stdlib" have richer API then the ABC. (e.g. comparison, +, *, etc...) They are "APIs that sequence may have" but not "APIs makes the type sequence." ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16728> _______________________________________