I've report http://bugs.python.org/issue16728 , but I am confused about what is the sequence now.
Glossary defines sequence as iteratable having __getitem__ and __len__. Objects doesn't have __iter__ is iterable when it having __getitem__.
Sequences also support slicing: a[i:j] selects all items with index *k*such that
*i* <= *k* < *j*. When used as an expression, a slice is a sequence of the same type. This implies that the index set is renumbered so that it starts at 0.
But I think this sentence explains about standard types and not definition of sequence.
This module provides *abstract base classes*http://docs.python.org/3/glossary.html#term-abstract-base-classthat can be used to test whether a class provides a particular interface;
for example, whether it is hashable or whether it is a mapping.
And collections.abc.Sequence requires "index()" and "count()".
What is the requirement for calling something is "sequence"?
Off Topc: Sequence.__iter__ uses __len__ and __getitem__ but default iterator uses only __getitem__. This difference is ugly.