[Python-Dev] Proposal for a new itertools function: iwindow

Raymond Hettinger raymond.hettinger at verizon.net
Sun May 28 05:04:11 CEST 2006

From: "Nick Coghlan" <ncoghlan at gmail.com>
> A python-dev Google search for "itertools window" found me your original 
> suggestion to include Jack Diedrich's itertools.window in Python 2.3 (which 
> was only deferred because 2.3 was already past beta 1 at that point).
> I couldn't find any discussion of the idea after that (aside from your 
> pointing out that you'd never found a real-life use case for the pairwise() 
> recipe in the docs, which is a basic form of windowing).
> One option would be to add a windowing function to the recipes in the 
> itertools docs. Something like:
> def window(iterable, window_len=2, window_step=1):
>     iterators = tee(iterable, window_len)
>     for skip_steps, itr in enumerate(iterators):
>         for ignored in islice(itr, skip_steps):
>             pass
>     window_itr = izip(*iterators)
>     if window_step != 1:
>         window_itr = islice(window_itr, step=window_step)
>     return window_itr

No thanks.  The resolution of this one was that windowing iterables is not a 
good idea.  It is the natural province of sequences, not iterables.  With 
sequences, it is a matter of using an index and offset.  With iterables, there 
is a great deal of data shifting.  Also note that some of the uses are subsumed 
by collections.deque().  In implementing a draft of itertools window, we found 
that the data shifting was unnatural and unavoidable (unless you output some 
sort of buffer instead of a real tuple).  Also, we looked at use cases and found 
that most had solutions that were dominated by some other approach.  The 
addition of a windowing tool would tend to steer people away from better 
solutions.  In short, after much deliberation and experimenting with a sample 
implementation, the idea was rejected.  Hopefully, it will stay dead and no one 
will start a cruscade for it simply because it can be done and because it seems 

The thought process was documented in a series of newsgroup postings:

The SF history is less informative because most of the discussions were held by 
private email:

If someone aspires to code some new itertools, I have approved two new ones, 
imerge() and izip_longest().  The pure python code for imerge() is at:

The impetus behind izip_longest() was expressed in a newsgroup thread, 
http://groups.google.com/group/comp.lang.python/browse_thread/thread/f424f63bfdb77c4/38c31991133757f7 . 
The discussion elicted neither broad support, nor condemnation.  Also, the use 
cases were sparse.  Ultimately, I was convinced by encountering a couple of 
natural use cases.  Another thought is that while other solutions are usually 
available for any given use case, there is a natural tendency to reach for a 
zip-type tool whenever presented with lock-step iteration issues, even when the 
inputs are of uneven length.  Note, that the signature for izip_longest() needs 
to include an optional pad value (defaulting to None) -- there are plenty of use 
cases where an empty string, zero, or a null object would be a preferred pad 



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