[Python-Dev] Re: PEP 322: Reverse Iteration (original) (raw)

Samuele Pedroni pedronis at bluewin.ch
Wed Nov 5 16:44:59 EST 2003

At 19:14 05.11.2003 +0100, Alex Martelli wrote:

On Wednesday 05 November 2003 05:34 pm, Samuele Pedroni wrote: ... > I think he was wondering whether people rely on > > enumerate([1,2]).next > i = enumerate([1,2]) > i is iter(i) > > working , vs. needing iter(enumerate([1,2]).next > > I think he was proposing to implement enumerate as > > class enumerate(object): > def init(self,iterable): > self.iterable = iterable > > def iter(self): > i = 0 > for x in self.iterable: > yield i,x > i += 1 > > def reversed(self): > rev = reversed(self.iterable) > try: > i = len(self.iterable)-1 > except (TypeError,AttributeError): > i = -1 > for x in rev: > yield i,x > i -= 1

Ah, I see -- thanks! Well, in theory you COULD add a 'next' method too: def next(self): self.iterable = iter(self.iterable) try: self.index += 1 except AttributeError: self.index = 0 return self.index, self.iterable.next() (or some reasonable optimization thereof:-) -- now reversed would stop working after any .next call, but that would still be OK for all use cases I can think of.

well, you would also get an iterator hybrid that violates:

""" Iterator objects also need to implement this method [iter]; they are required to return themselves. """

http://www.python.org/doc/2.3.2/ref/sequence-types.html#l2h-234

what one could do is:

class enumerate(object): def init(self,iterable): self.iterable = iterable self.forward = None self.index = 0

 def __iter__(self):
   return self

def next(self):
    if not self.forward:
       self.forward = iter(self.iterable)
    i = self.index
    self.index += 1
   return i, self.forward.next()

def __reversed__(self):
  if self.forward:
     raise Exception,...

 rev = reversed(self.iterable)
 try:
    i = len(self.iterable)-1
 except (TypeError,AttributeError):
    i = -1
 for x in rev:
    yield i,x
    i -= 1

but is still an hybrid, setting a bad precedent of trying too hard to attach reversed to an iterator, making enumerate just an iterable is not backward compatible but is a bit saner although it does not feel that natural either.

regards.

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