Real-world use cases for map's None fill-in feature?

rurpy at rurpy at
Thu Jan 12 18:47:09 EST 2006

"Raymond Hettinger" <python at> wrote:
> [rurpy at]
> > > > How well correlated in the use of map()-with-fill with the
> > > > (need for) the use of zip/izip-with-fill?
> [raymond]
> > > Close to 100%.  A non-iterator version of izip_longest() is exactly
> > > equivalent to map(None, it1, it2, ...).
> [rurpy at]
> > If I use map()
> > I can trivially determine the arguments lengths and deal with
> > unequal length before map().  With iterators that is more
> > difficult.  So I can imagine many cases where izip might
> > be applicable but map not, and a lack of map use cases
> > not representative of izip use cases.
> You don't seem to understand what map() does.  There is no need  to
> deal with unequal argument lengths before map(); it does the work for
> you.  It handles iterator inputs the same way.  Meditate on this:
>     def izip_longest(*args):
>         return iter(map(None, *args))
> Modulo arbitrary fill values and lazily evaluated inputs, the semantics
> are exactly what is being requested.  Ergo, lack of use cases for
> map(None,it1,it2) means that izip_longest(it1,it2) isn't needed.

"lazily evaluated inputs" is exactly what I was pointing
out and what make your izip_longest() above not the
same as map(None,...), and hence, your conclusion
invalid.  Specifically....

def izip_longest(*args):
    return iter(map(None, *args))
f1 = file ("test.dat")
f2 = file ("test.dat")
it = izip2 (f1, f2)
while 1:
    h1, h2 = ()
    print h1.strip(), h2

izip2() in the above code is a "real" izip_longest
based on a version posted in this thread.

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Replace izip2 in the above code with your izip_longest
[wait, wait, wait,... after a few minutes type ^c, nothing
happens, close window].

I don't think your izip_longest is at all equivalent to
the proposed izip, and thus there may well be uses
cases for izip that aren't represented by imap(None,...)
use cases, which is what I said.  That is, I might have
a use case for izip which I would never even consider
map() for.

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