Python 3 preparation: Iterator protocol uses .next() in Py2 but __next__() in Py3

[Wilfried Luebbe]

When defining a custom iterator one has to define a method that returns the next item. In Python 3 this method is called next() while in Python 3 this is the special method __next__().

More information can be found in ​http://python-future.org/compatible_idioms.html#custom-iterators and ​http://python-future.org/what_else.html#custom-iterators.

Remark: To advance some iterator it in Py2 often this method is called directly it.next(). But since Python 2.6 there is the builtin function next() which is compatible between Py2 and Py3: in Py2 it calls the iterator method next() while in Py3 it calls the special method __next__(). Ticket #16075 addresses the (stage 1) conversion from it.next() to next(it).

This ticket is tracked as a dependency of meta-ticket #16052.

[aapitzsch]

New commits:

​840f968

rename next() to __next__() and keep next() as alias

[Frédéric Chapoton]

Are you sure that all these things are really iterators ?

[aapitzsch]

Replying to chapoton:

Are you sure that all these things are really iterators ?

Yes.

[Frédéric Chapoton]

I do not quite agree with that. The next methods in permutation and partition are not really iterators (they are implemented on elements, not on a set, and they return False when they cannot return something), and should in fact not be called using next(p) but rather as a method p.next(). Of course, it may be convenient to use next(p) as a shortcut, but this is not the usual semantics of iterators, so I would rather deprecate that.

[Frédéric Chapoton]

ok, let us do that, despite the fact that some are not really iterators.