[Python-ideas] + operator on generators

[Steven D'Aprano]

On Sat, Jul 01, 2017 at 01:35:29AM -0500, Wes Turner wrote:
> On Saturday, July 1, 2017, Steven D'Aprano <steve at pearwood.info> wrote:
> 
> > On Fri, Jun 30, 2017 at 01:09:51AM +0200, Jan Kaliszewski wrote:
> >
> > [...]
> >
> > But the more I think about it the more I agree with Nick. Let's start
> > by moving itertools.chain into built-ins, with zip and map, and only
> > consider giving it an operator after we've had a few years of experience
> > with chain as a built-in. We might even find that an operator doesn't
> > add any real value.
> 
> 
> - Would that include chain.from_iterable?

Yes.

> - So there's then a new conditional import (e.g. in a compat package)? What
> does this add?

try:  chain
except NameError:  from itertools import chain

Two lines, if and only if you both need chain and want to support 
versions of Python older than 3.7.

There's no need to import it if you aren't going to use it.


> > > ¹ Preferably using the existing `yield from` mechanism -- because, in
> > > case of generators, it would provide a way to combine ("concatenate")
> > > *generators*, preserving semantics of all that their __next__(), send(),
> > > throw() nice stuff...
> >
> > I don't think that would be generally useful.
> 
> Flatten one level?

Flattening typically applies to lists and sequences.

I'm not saying that chain shouldn't support generators. That would be 
silly: a generator is an iterable and chaining supports iterables. I'm 
saying that it wouldn't be helpful to require chain objects to support 
send(), throw() etc.

> > If you're sending values
> > into an arbitrary generator, who knows what you're getting? chain() will
> > operate on arbitrary iterables, you can't expect to send values into
> > chain([1, 2, 3], my_generator(), "xyz") and have anything sensible
> > occur.
> 
> 
> - is my_generator() mutable (e.g. before or during iteration)?

It doesn't matter. Sending into a chain of arbitrary iterators isn't a 
useful thing to do.


> - https://docs.python.org/2/reference/expressions.html#generator.send

Why are you linking to the 2 version of the docs? We're discusing a 
hypotheticial new feature which must go into 3, not 2.


-- 
Steve