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

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
   else:
      # 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

--jw