[Python-iterators] RE: PEP 234: Iterators

[Guido van Rossum]

> [Peter Caven]
> > ...
> > I would really like to be able to write a loop over only the keys,
> > values or items in a dict that pass some test (filter):
> >
> > for KeyFilter(),value  in dict:
> >   ...do something with the values that have keys that pass the filter.
> >
> > I know this idea is half-baked.  Is there some other way of elegantly
> > doing this without doing an explicit test inside the loop?

[Tim]
> List comprehensions support iteration + filtering directly, but materialize
> the whole result in one gulp.  The following works fine in current CVS
> Python, expoiting the proposed new 2.2 iterator gimmicks:
> 
[code omitted]
> 
> d = {1:2, 2:3, 3:4, 4:5}
> 
> for k in filteriter(lambda k: k & 1 == 0, d):
>     print "even key", k  # prints 2 and 4
> 
> for v in filteriter(lambda k: k & 1, d.itervalues()):
>     print "odd value", v # prints 3 and 5
> 
> evenkeys = filteriter(lambda k: k & 1 == 0, d)
> bigevenkeys  = filteriter(lambda k: k > 2, evenkeys)
> 
> for whee in bigevenkeys:
>     print whee # prints 4
> 
> I think it should be clear that an iterator can support any
> computation whatsoever, including filtering, transforming,
> stuttering, ..., whatever you can dream up.  Plus they can be
> chained together (as in bigevenkeys above).

With the latest CVS of 2.2, you don't have to write the filteriter
class -- the built-in function filter() can do this:

    >>> for k in filter(lambda k: k & 1 == 0, d):
    ... print k
    ... 
    4
    2
    >>> 

--Guido van Rossum (home page: http://www.python.org/~guido/)