[Python-ideas] Inline assignments using "given" clauses
Eric V. Smith
eric at trueblade.com
Sun May 13 06:45:29 EDT 2018
On 5/13/18 1:05 AM, Chris Angelico wrote:
> 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
>>>> its rejection? I'm sure there's one in the many in this discussion but
>>>> not found it yet.
>>> I'm not sure which version you're looking at, so there's the rejections of
>> 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
>> 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.
A little more detail:
The first one is partially:
target = expr.__enter__()
The second one is partially:
target = expr
For many objects, obj.__enter__() just returns obj, it often looks like
these two statements do the same thing, but they do not. Chris's
concern, which I share, is that users wouldn't know why these are
different, and why the second on works for some objects but not others.
I agree the PEP could use more detail in explaining this particular issue.
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