Accessing next/prev element while for looping

[Alex Martelli]

Steven D'Aprano <steve at REMOVETHIScyber.com.au> wrote:

> I can't speak for others, but I've never come upon a situation where I
> needed to access the element before and the element after the current one.
> 
> [thinks...] Wait, no, there was once, when I was writing a parser that
> iterated over lines. I needed line continuations, so if the line ended
> with a backslash, I needed to access the next line (and potentially the
> line after that, and so forth indefinitely). I dealt with that by keeping
> a cache of lines seen, adding to the cache if the line ended with a
> backslash.

In Python, that would be an excellent example use case for a generator
able to "bunch up" input items into output items, e.g.:

def bunch_up(seq):
  cache = []
  for item in seq:
    if item.endswith('\\\n'):
      cache.append(item[:-2])
    else:
      yield ''.join(cache) + item
      cache = []

  if cache:
     raise ValueError("extra continuations at end of sequence")

[[or whatever you wish to do instead of raising if the input sequence
anomalously ends with a ``to be continued'' line]].


Alex