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Re: Best way to check that you are at the beginning (the end) of an iterable?

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On 09/07/11 18:22, Laurent wrote:
> Anyway I was just asking if there is something better than
> enumerate. So the answer is no? The fact that I have to create
> a tuple with an incrementing integer for something as simple
> as checking that I'm at the head just sounds awfully
> unpythonic to me.

I've made various generators that are roughly (modulo 
edge-condition & error checking) something like

  def with_prev(it):
    prev = None
    for i in it:
      yield prev, i
      i = prev

  def with_next(it):
    prev = it.next()
    for i in it:
      yield prev, i
      prev = i
    yield prev, None

which can then be used something like your original

   for cur, next in with_next(iterable):
     if next is None:
       do_something_with_last(cur)
     else:
       do_regular_stuff_with_non_last(cur)

   for prev, cur in with_prev(iterable):
     if prev is None:
       do_something_with_first(cur)
     else:
       do_something_with_others(cur)

If your iterable can return None, you could create a custom 
object to signal the non-condition:

   NO_ITEM = object()

and then use NO_ITEM in place of "None" in the above code.

-tkc

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Thread

Best way to check that you are at the beginning (the end) of an iterable? Laurent <laurent.payot@gmail.com> - 2011-09-07 14:35 -0700
  Re: Best way to check that you are at the beginning (the end) of an iterable? Cameron Simpson <cs@zip.com.au> - 2011-09-08 08:48 +1000
    Re: Best way to check that you are at the beginning (the end) of an iterable? Laurent <laurent.payot@gmail.com> - 2011-09-07 16:22 -0700
    Re: Best way to check that you are at the beginning (the end) of an iterable? Laurent <laurent.payot@gmail.com> - 2011-09-07 16:22 -0700
      Re: Best way to check that you are at the beginning (the end) of an iterable? Cameron Simpson <cs@zip.com.au> - 2011-09-08 10:23 +1000
        Re: Best way to check that you are at the beginning (the end) of an iterable? Laurent <laurent.payot@gmail.com> - 2011-09-07 17:53 -0700
        Re: Best way to check that you are at the beginning (the end) of an iterable? Laurent <laurent.payot@gmail.com> - 2011-09-07 17:53 -0700
        Re: Best way to check that you are at the beginning (the end) of an iterable? Chris Torek <nospam@torek.net> - 2011-09-08 14:21 +0000
          Re: Best way to check that you are at the beginning (the end) of an iterable? Cameron Simpson <cs@zip.com.au> - 2011-09-09 08:39 +1000
      Re: Best way to check that you are at the beginning (the end) of an iterable? Tim Chase <python.list@tim.thechases.com> - 2011-09-07 19:01 -0500
        Re: Best way to check that you are at the beginning (the end) of an iterable? Laurent <laurent.payot@gmail.com> - 2011-09-07 18:08 -0700
        Re: Best way to check that you are at the beginning (the end) of an iterable? Laurent <laurent.payot@gmail.com> - 2011-09-07 18:08 -0700
      Re: Best way to check that you are at the beginning (the end) of an iterable? Terry Reedy <tjreedy@udel.edu> - 2011-09-07 21:06 -0400
    Re: Best way to check that you are at the beginning (the end) of an iterable? Peter Otten <__peter__@web.de> - 2011-09-09 13:04 +0200
      Re: Best way to check that you are at the beginning (the end) of an iterable? Chris Angelico <rosuav@gmail.com> - 2011-09-09 21:30 +1000
  Re: Best way to check that you are at the beginning (the end) of an iterable? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-09-08 10:24 +1000
    Re: Best way to check that you are at the beginning (the end) of an iterable? Terry Reedy <tjreedy@udel.edu> - 2011-09-07 21:08 -0400
    Re: Best way to check that you are at the beginning (the end) of an iterable? Laurent <laurent.payot@gmail.com> - 2011-09-07 18:05 -0700
  Re: Best way to check that you are at the beginning (the end) of an iterable? Miki Tebeka <miki.tebeka@gmail.com> - 2011-09-07 17:24 -0700
    Re: Best way to check that you are at the beginning (the end) of an iterable? Laurent <laurent.payot@gmail.com> - 2011-09-07 18:06 -0700
    Re: Best way to check that you are at the beginning (the end) of an iterable? Chris Rebert <clp2@rebertia.com> - 2011-09-07 19:27 -0700

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