INADA Naoki added the comment: Thanks, Nick. I see that the "sequence" doesn't have strict definition. Though, I think collections.abc module's document should describe this manner. For example: "But checking type with these abc may be too strict for most case. For example, some user defined sequence class may not cover :class:`Sequence` abc. Using :term:`EAFP` manner is preferred in Python." ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16728> _______________________________________