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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 9/7/2011 8:23 PM, Cameron Simpson wrote:
> On 07Sep2011 16:22, Laurent<laurent.payot@gmail.com>  wrote:
> | I totally understand the performance issue that an hypothetical
> | "istail" would bring, even if I think it would just be the programmer's
> | responsibility not to use it when it's not certain that an end can
> | be detected.
>
> The trouble with these things is that their presence leads to stallable
> code, often in libraries. Let the programmer write code dependent on
> istail() without thinking of the stall case (or even the gratuitous
> execution case, as in a generator with side effects in calling .next())
> and have that buried in a utilities function.
>
> Facilities like feof() in C and eof in Pascal already lead to lots of
> code that runs happily with flat files and behaves badly in interactive
> or piped input. It is _so_ easy to adopt a style like:
>
>    while not eof(filehandle):
>      line = filehandle.nextline()
>      ...
>
> that is it often thought that having offered the eof() function is a
> design error. (Of course in the example above the usual python idiom
> would win out from existing habit, but there are plenty of other
> situations where is would just be _easy_ to rely of istail() in whatever
> form.)
>
> | But I don't see why *adding* something like "ishead" would be so bad
> | (at worse by using a boolean somewhere as you mentioned).
>
> It is not awful, but as remarked:
>    - extra storage cost to _every_ iterable, for a rarely used facility
>    - extra runtime cost to maintain the state
>    - _retroactive_ burden on _every_ iterator implementation presently
>      existing; every iterator sudden needs to implement and offer this
>      extra facility to be generate purpose use
>    - it is easy to provide the facility on the rare occasions when it is
>      needed
>
> Personally, I think point 3 above is the killer and 1 and 2 are serious
> counter arguments.

The iterator protocol is intentionally as simple as sensibly possible.

> | 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.
>
> You can just use a boolean if you like. I have plent of loops like:
>
>    first = true
>    for i in iterable:
>      if first:
>        blah ...
>      ...
>      first = False
>
> Cheap and easy. Cheers,

Or grab and process the first item separately from the rest.

it = iter(iterable)
try:
   first = next(it)
   <process first item>
except StopIteration:
   raise ValueError("Empty iterable not allowed")
for i in it:
   <process non-first item>

-- 
Terry Jan Reedy

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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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