[Python-Dev] Half-baked proposal: * (and **?) in assignments

John Williams jrw@pobox.com
Mon, 25 Nov 2002 14:58:01 -0600

Alex Martelli wrote:
 > What WOULD be intolerable, it appears to me, would be to *require*
 > that user-coded iterators (classes exposing currently-suitable
 > __iter__ and next methods) MUST subclass iter.  That would break
 > existing, running code, and go against the grain of Python, which
 > nowhere else imposes such requirements.  Having a (mix-in?) class that
 > iterators COULD subclass (as Brent suggests) is one thing; for Python
 > to REQUIRE such subtyping (as Armin appears to wish could be done) is
 > quite another.

What if you turn this around and place the burden on the Python system? 
  Make "iter" a class rather than a function, and ensure that 
iter.__new__ always returns a subclass of "iter" like this (untested code):

   class iter(object):

     def __new__(cls, iterable):
       userIterator = iterable.__iter__()
       if isinstance(userIterator, iter):
          # Just like today's "iter" function.
          return userIterator
          # Build a wrapper.
          wrapper = object.__new__(iter)
          wrapper.next = userIterator.next
          if hasattr(userIterator, "__iter__"):
            wrapper.__iter__ = userIterator.__iter__
          return wrapper

      def next(self):
        raise NotImplementedError

      def __iter__(self):
        return self

      # arbitrary new convenience methods here