[Python-Dev] PEP 572: Assignment Expressions

[Nick Coghlan]

On 19 April 2018 at 02:38, Brett Cannon <brett at python.org> wrote:
> I'm also -1.
>
> I understand the usefulness of the construct in languages where block scopes
> make having this kind of expression assignment in e.g. an 'if' guard useful.
> But for Python and it's LGB scoping -- although I think we need to add an
> "N" for "non-local" :) -- this is syntactic sugar and I don't see enough
> wide benefit on top of the potential ugliness/misuse of it to warrant the
> cognitive overhead of adding it.
>
> Although, as usual, Chris, great PEP! :)

Aye, tremendous work on the PEP Chris - assignment expressions are an
idea that has come up many, many times on python-ideas, and it's great
to finally have a consolidated proposal that attempts to make the best
case it can for the idea.

Unfortunately, I still have to admit that even I'm -0 on the idea of
actually adding it to the language (despite helping it to reach a
plausibly acceptable state), primarily on "more than one way to do it"
grounds:

* assignment statements and top-level assignment expressions would be
syntactically distinct, but semantically identical
* for the if statement use case, pre-assignment of the re-used part of
the condition expression already works fine
* for the while loop use case, I think my discussion with Tim about
tuple unpacking shows that the loop-and-a-half construct won't be
going anywhere, so users would still end up needing to learn both
forms (even for new code)

The comprehension/genexp use case still seems like the most promising
candidate for a reasonable justification for the extra complexity, but
there's also a solid argument that if a comprehension is complex
enough that the lack of assignment expressions is causing problems,
then it's also complex enough that it's reasonable to require
extracting the algorithm into a named generator function in order to
access assignment statements.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia