[Python-Dev] Informal educator feedback on PEP 572 (was Re: 2018 Python Language Summit coverage, last part)

[Chris Angelico]

On Sat, Jun 23, 2018 at 1:48 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> I can't say I've done a broad survey, but the third-party documentation
> I've read on comprehensions typically glosses over the scoping issues
> without mentioning them. To the extent that scoping is even hinted at,
> comprehensions are treated as expressions which are exactly equivalent
> to re-writing them as a for-loop in the current scope.

Even first-party documentation elides that distinction. The same
inaccurate-but-simple equivalency - even using the word "equivalent" -
comes up here:

https://docs.python.org/3/howto/functional.html?highlight=equivalent#generator-expressions-and-list-comprehensions

So I'm very sympathetic to the desire to have assignment expressions
inside comprehensions behave like assignment expressions outside
comprehensions. The trouble is that they are then _not_ the same as
other names inside comprehensions. One way or another, there's a
confusing distinction, especially at class scope. Unless this comes
with an actual semantic change that affects existing code, there is
going to be a bizarre disconnect *somewhere*, and it's just a matter
of where.

ChrisA