[Python-ideas] Aid reiteration with new class: gfic

Paul Moore p.f.moore at gmail.com
Fri Jun 19 22:13:17 CEST 2009

2009/6/19 John Graham <john.a.graham at gmail.com>:
> Exactly how does itertools.tee not cover this use case?

I've been wondering the same sort of thing. But if you're going to
sequentially run through the iterator twice

    for i in it:
        do something
    for i in it:
        do something else

then itertools.tee is documented as being basically equivalent to, but
worse than, list(): "In general, if one iterator is going to use most
or all of the data before the other iterator, it is faster to use
list() instead of tee()".

So the question becomes, how does list() not cover this use case.

The only answer I can see is that the proposal avoids allocating
temporary storage for all of the values generated. Consider
xrange(1000000) - if you can call xrange twice, rather than saving a
million values, you've saved a lot of memory.

But this only applies when you're writing functions designed to take
extremely general iterators. Someone writing a function which can take
essentially arbitrary iterators, and which has to handle unboundedly
large input gracefully, probably has worse problems than the lack of
standard library support for something they can code themselves fairly


More information about the Python-ideas mailing list