[Python-Dev] itertools addition: getitem()

[Kevin Jacobs <jacobs@bioinformed.com>]

On 7/8/07, Guido van Rossum <guido at python.org> wrote:
>
> Ahem. I hope you have a better use case for getitem() than that
> (regardless of the default issue). I find it clearer to write that as
>
> try:
>   compid = root[ns.company_id].next()
> except StopIteration:
>   compid = None
> else:
>   compid = int(compid)
>
> While this is more lines, it doesn't require one to know about
> getitem() on an iterator. This is the same reason why setdefault() was
> a mistake -- it's too obscure to invent a compact spelling for it
> since the compact spelling has to be learned or looked up.
>


Apropos of this discussion, I've occasionally wanted a faster version of the
following:

_nothing=object()

def nth_next(seq,n,default=_nothing):
  '''
  Return the n'th next element for seq, if it exists.

  If default is specified, it is return when the sequence is too short.
  Otherwise StopIteration is raised.
  '''
  try:
    for i in xrange(n-1):
      seq.next()
    return seq.next()
  except StopIteration:
    if default is _nothing:
      raise
    return default

The nice thing about this function is that it solves several problems in
one: extraction of the n'th next element, testing for a minimum sequence
length given a sentinel value, and just skipping n elements.  It also leaves
the sequence in a useful and predictable state, which is not true of the
Python-version getitem code.  While cute, I can't say if it is worthy of
being an itertool function.

Also vaguely apropos:

def ilen(seq):
  'Return the length of the hopefully finite sequence'
  n = 0
  for x in seq:
    n += 1
  return n

Why?  Because I find myself implementing it in virtually every project.
Maybe I'm just an outlier, but many algorithms I implement need to consume
iterators (for side-effects, obviously) and it is sometimes nice to know
exactly how many elements were consumed.

~Kevin
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