This is, I think, largely a result of Python not idiomatically using mixins as much as Ruby (since it has multiple inheritance, it supports them, and they are sometimes used, but Ruby leans into them hard in the core.)

In the Ruby way, with otherwise similar setup, you would have a mixin that provided __iter__() given __len__() and __getitem__(), and classes that had the last two but wanted the behavior of the first would just include that mixin class as a parent, rather than having a separate branch in iter() to handle the different cases. (Ironically, its one way that Ruby is less TMTOWTDI than Python is.)