[Python-ideas] Inline assignments using "given" clauses

[Chris Angelico]

On Sun, May 13, 2018 at 2:58 PM, Cameron Simpson <cs at cskk.id.au> wrote:
> On 13May2018 14:23, Chris Angelico <rosuav at gmail.com> wrote:
>>
>> On Sun, May 13, 2018 at 2:05 PM, Cameron Simpson <cs at cskk.id.au> wrote:
>>>
>>> Could someone point me to a post which nicely describes the rationale
>>> behind
>>> its rejection?  I'm sure there's one in the many in this discussion but
>>> I've
>>> not found it yet.
>>
>>
>>
>> https://www.python.org/dev/peps/pep-0572/#special-casing-conditional-statements
>> https://www.python.org/dev/peps/pep-0572/#alternative-spellings
>>
>> I'm not sure which version you're looking at, so there's the rejections of
>> both.
>
>
> I meant the latter, but I'd already looked at that part of the PEP and found
> its explaination... unfulfilling. It says:
>
>  EXPR as NAME:
>
>    stuff = [[f(x) as y, x/y] for x in range(5)]
>
>  Since EXPR as NAME already has meaning in except and with statements (with
>  different semantics), this would create unnecessary confusion or require
>  special-casing (eg to forbid assignment within the headers of these
>  statements).
>
> All you need to disambiguate, say:
>
>  with expr as expr_as as with_as:
>
> is to require parentheses for (expr as exp_as) if someone wanted that
> complications (assuming that is even necessary - it seems unambiguous to my
> eye already, unless the token lookahead requirement in Python's grammar
> prevents that.
>
> So I'd hoped for a post to the list discussing this aspect and outlining why
> it was considered unsatisfactory.

There were a large number of posts, so I can't really point to one of
them. The problem isn't the double-as case that you describe; it's
that these two are ALMOST identical:

with expr as target:
with (expr as target):

In fact, they are functionally identical for many situations - "with
(open(...) as target):" is exactly the same as the form without the
parens. It'd make for data-dependent bugs, which are a really REALLY
bad idea.

ChrisA