[David Abrahams]
I keep running into the problem that there is no reliable way to introspect about whether a type supports multi-pass iterability (in the sense that an input stream might support only a single pass, but a list supports multiple passes). I suppose you could check for __getitem__, but that wouldn't cover linked lists, for example.
Can anyone channel Guido's intent for me? Is this an oversight or a deliberate design decision? Is there an interface for checking multi-pass-ability that I've missed?
The language makes no such distinctions. If an app wants to make them, it's up to the app to implement them. Likewise for a way to tell a multipass iterator to "start over again". The Python iteration protocol has only two methods, .next() to get "the next" item, and .iter() to return self; given a random iterator, those are the only things you can rely on.