Do you have real-world use cases for map's None fill-in feature?

Raymond Hettinger python at
Tue Jan 10 03:56:35 EST 2006

[Raymond Hettinger]
> > I am evaluating a request for an alternate version of itertools.izip()
> > that has a None fill-in feature like the built-in map function:
> >
> > >>> map(None, 'abc', '12345')   # demonstrate map's None fill-in feature

[Paul Rubin]
> I think finding different ways to write it was an entertaining
> exercise but it's too limited in usefulness to become a standard
> feature.

Makes sense.

> I do think some idiom ought to develop to allow checking whether an
> iterator is empty, without consuming an item.  Here's an idea:
> introduce something like
>    iterator = check_empty(iterator)

There are so many varieties of iterator that it's probably not workable
to alter the iterator API for all of the them.  In any case, a broad
API change like this would need its own PEP.

> There are some obvious problems with the above:
>  1) the new iterator should support all of the old one's attributes,
>     not just inherit its operations
>  2) In the case where the old iterator is already buffered, the
>     constructor should just peek at the lookahead instead of making
>     a new object.  That means that checking an iterator multiple times
>     won't burn more and more memory.
> Maybe there is some way of doing the above with metaclasses but I've
> never been able to wrap my head around those.

Metaclasses are unlikely to be of help because there are so many,
unrelated kinds of iterator -- most do not inherit from a common


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