[Python-Dev] Re: PEP 322: Reverse Iteration
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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