From: Alex Martelli

On 2004 Feb 25, at 12:13, Dmitry Vasiliev wrote:

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Raymond Hettinger wrote:

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Currently, writelines() does not add trailing line separators.. This is fine when working with readlines() but a PITA in other situations. If we added an optional separator argument, it would be easier to add newlines and we would gain some of the flexibility of str.join() at full C speed.

Maybe not a separator but suffix, so newline will be added to last line too?

Good point. And while a separator would be a slight nuisance to express otherwise, a "suffix" isn't -- it seems to me that f.writelines(x+'\n' for x in mylines) is a rather good way of expressing "suffix each line with a \n". I don't think this suffixing operation is so widely more important than other elaborations on items of mylines to make it worth specialcasing into a writelines argument [if anything, f.writelines(str(x) for x in mylines) would be the one elaboration that seems to me to be by far the most frequent -- still not worth specialcasing though, IMHO].

Maybe there's a useful itertool lurking in here. I'm thinking of something that takes a list of iterables, and generates elements from each in turn, like a flattened izip: def interleave(*iterables): iterables = [iter(it) for it in iterables] while 1: # Let StopIteration fall through, but make sure that # each iterable generates the same number of results vals = [it.next() for it in iterables] for val in vals: yield val Then, we're talking about f.writelines(interleave(mylines, repeat('\n'))) [or if you want separators rather than suffixes, use f.writelines(islice(interleave(repeat('\n'), mylines), 2, None)) instead] Frankly, it's not an obvious win for the writelines case, but I was surprised that there was nothing like interleave in itertools, and that I couldn't find an easy way to write such a thing. Maybe a flatten itertool would be more generally useful - def flatten(iterable): for it in iterable: for val in it: yield val then interleave(*iterables) is flatten(izip(*iterables))... Thoughts? If not added as functions, would these be useful additions to the documentation? Paul.