[Python-Dev] itertools predicates

Raymond Hettinger raymond.hettinger at gmail.com
Fri Nov 4 13:29:27 EDT 2016

> On Nov 3, 2016, at 3:51 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> Before there can be a solution, there first has to be a problem that 
> needs solving. "Lack of consistency" is not necessarily a problem. The 
> intertools functions are quite different, they do different things with 
> different APIs. The question here should not be "why don't these 
> functions take None as an argument?", rather it should be "why should 
> these functions take None as an argument?".

I concur with Steven who articulated the issue perfectly.  There isn't an actual problem here that needs to be solved (i.e. not a single user report in 15 years indicating that the API wasn't meeting the needs for real use-cases).  I'm disinclined to churn the API unless there is a real need.

FWIW, groupby() has the predicate as an optional argument so that you can write groupby('aaabbc') and have it group by value (much like the key-function on sorted() is optional).   The two filter variants allow None as the first argument only for historical reasons -- once "bool" came along, it would have been better to write filter(bool, someiterable) in preference to using None which is less clear about its intention.  The takewhile/dropwhile tools didn't have the same constraint to match a historical API, so there was an opportunity to have a clearer API with a simpler signature.

As Terry suggested, if you have other itertools feature requests, please put them on the tracker an assign them to me.

Thank you,


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