[Python-ideas] Slicing and Chainging iterables.

Steven D'Aprano steve at pearwood.info
Sat Jan 3 16:28:44 CET 2015

On Sat, Jan 03, 2015 at 03:37:03PM +0200, yotam vaknin wrote:
> I would like to purpose that in python 3 it will be easy to chain and slice
> Iterators just like you can add and slice lists and tuples easily.
> I think there could be 2 methods to do this:
> 1. Implementing '+' and slicing ([1:2:3]) for iterators and generators by
> default, Resulting in  Itertool's chain(a,b) and islice(a,b,c) respectively.
> 2. Having itertool's chain and islice imported by default.

The iterator protocol is intentionally simple. To create an iterator, 
you need to only define two methods:

(1) __iter__, which returns the instance itself;

(2) __next__, which returns the next item and raises StopIterator when 

With your proposal, you would have to define two or three more methods:

(3) __add__ and possibly __radd__, to chain iterators;

(4) __getitem__, for slicing.

There is nothing stopping you from adding these methods to your own 
iterator classes, but with your proposal that would be compulsory for 
all iterators.

> I think since python 3 return zip,map, dict.items and so on as iterators,
> it makes working with those kind of objects more difficult without having
> these methods around. And since those methods are important enough to have
> for lists, it seems important enough for iterators too.

It only takes a single line of code to get iterator chaining and 

from itertools import chain, islice

And now you can chain and slice any iterator, regardless of where it 
came from. I don't think that is difficult.

*If* iterators where instances of a concrete base class, then it would 
make sense to add chain and slice methods to that base class so all 
other iterators could inherit from it. But they aren't, iterators in 
Python are values which obey a protocol, not inheritence. That means 
that a functional approach, like itertools, is more appropriate.

I can see the appeal of using + and [a:b:c] syntax instead of function 
syntax chain() and islice(), but I don't think the advantage is enough 
to out-weigh the costs.


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