Iterator length

[Steven D'Aprano]

On Fri, 19 Jan 2007 05:04:01 -0800, bearophileHUGS wrote:

> Steven D'Aprano:
>> > s = "aaabbbbbaabbbbbb"
>> > from itertools import groupby
>> > print [(h,leniter(g)) for h,g in groupby(s)]
>>
>> s isn't an iterator. It's a sequence, a string, and an iterable, but not
>> an iterator.
> 
> If you look better you can see that I use the leniter() on g, not on s.
> g is the iterator I need to compute the len of.


Oops, yes you're right. But since g is not an arbitrary iterator, one can
easily do this:

print [(h,len(list(g))) for h,g in groupby(s)]

No need for a special function.



>> I hope you know what sequences and strings are :-)
> 
> Well, I know little still about the C implementation of CPython
> iterators :-)
> 
> But I agree with the successive things you say, iterators may be very
> general things, and there are too many drawbacks/dangers, so it's
> better to keep leniter() as a function separated from len(), with
> specialized use.

I don't think it's better to have leniter() at all. If you, the iterator
creator, know enough about the iterator to be sure it has a predictable
length, you know how to calculate it. Otherwise, iterators in general
don't have a predictable length even in principle.



-- 
Steven.