On Thu, Jan 26, 2017 at 4:09 AM, Ivan Levkivskyi <levkivskyi@gmail.com> wrote:

Concerning list/set/dict comprehensions, I am much more in favor of making comprehensions simply equivalent to for-loops (more or less like you proposed using yield from). The only reason to introduce auxiliary function scope was to prevent the loop variables from leaking outside comprehensions.
Formally, this is indeed backward incompatible, but I doubt many people depend on the current counter-intuitive behavior.

Concerning generator expressions, probably it is indeed better to simply prohibit yield inside them.

Thank you to everyone who responded to my post and provided excellent analysis.

For Python, I don't know what the best way to proceed is:

OPTION 1

========

Make a SyntaxError:  [(yield 1) for x in range(10)] 

and update the documentation to explain that this is an invalid construct.

This would have certainly helped me identify the source of the problem as I tried porting buildbot 0.9 to Python 3.

However, while not very common, there is Python 2.x code that uses that.

I found these cases in the buildbot code which I changed so as to work on Python 2 and 3:

https://github.com/buildbot/buildbot/pull/2661

https://github.com/buildbot/buildbot/pull/2673

 OPTION 2

=========

Make this return a list on Python 3, like in Python 2:  [(yield 1) for x in range(10)] 

As pointed out by others on the this mailing list, there are some

problems associated with that.  I don't know if there are many Python 2 codebases out there

with this construct, but it would be nice to have one less Python 2 -> 3 porting gotcha.

I'm OK with either approach.  Leaving things the way they are in Python 3 is no good, IMHO.