[Python-Dev] Confusing listreverseiterator Behavior (original) (raw)

Armin Ronacher armin.ronacher at active-4.com
Tue Aug 26 22:04:03 CEST 2008

Jeff Hall <hall.jeff gmail.com> writes:

reversed( seq) Return a reverse iterator. seq must be an object which supports the sequence protocol (the len() method and the getitem() method with integer arguments starting at 0). New in version 2.4. the above appears to only be true for lists. For tuples and strings it creates a reverse OBJECT which behaves slightly differently (notably by not including a len() method as you noticed) I can't find how to actually create a "tuplereverseiterator" or "stringreverseiterator" objects... nor does there appear to be a way to create a "reversed" object from a list... That's an implementation detail what exactly the reverse iterator is. The same applies to iter() calls. iter("foo") for example returns a iterator type, iter([]) returns a list iterator. The thing you quoted above is the requirement for the object that you pass to reverse(), not the object returned which is some kind of iterator that happens to iterate over the sequence in reverse.

The problem I personally have with it is that the listreverseiterator is the only iterator in the standard library that changes the length during iteration and that it's the only reverse iterator that has a length. Even more stunning as the normal iterator doesn't have one.

Regards, Armin

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