[Python-ideas] PEP 572: Assignment Expressions (post #4)

Thautwarm Zhao yaoxiansamma at gmail.com
Thu Apr 12 14:01:38 EDT 2018 

 > > Makes sense. However, couldn't you prevent that by giving with
> priority over the binding ? As in "(with simple_cm) as value", where
> > we consider the "as" as binding operator instead of part of the with
> > statement ? Sure, you could commit suicide by parenthesis, but by
> > default it'd do exactly what the "with simple_cm as value" currently
> > does. This does require use of as instead of :=, though. (which was
> > the point I was trying to make, apologies for the confusion)
>
> Does "(with simple_cm) as value" means "with (simple_cm as value)"?
> If so, it's impossible to let the priority of "with ... as ..." over `as`
binding.
>
> This is the grammar  of current syntax related to with statement:
>
> with_stmt: 'with' with_item (',' with_item)* ':' suite
> with_item: test ['as' expr]
>
> If `as` binding could be used in a general expression, just as
> `test` is the top of expression, an expression using `as` binding must be
in the structure > `test`.
> In other words, if you write
>
> with expr as name:
>     # do stuff
>
> Without doubt it's equivalent to `with (expr as name)`.
>
> Or you want to completely change the grammar design of CPython :)
>
> thautwarm

Additionally, here is an evidence.

I've just had a look at Chris Angelico's implementation about expression
assignment, it's  cool, however the problem is still raised.

https://github.com/Rosuav/cpython/blob/0f237048b7665720b5165a40de0ed601c1e82c39/Grammar/Grammar

`as` binding is added at line 111, obviously you cannot separate it from
the `test` structure(because `test` is the top expr).

testlist_comp: (test|star_expr) ( comp_for | 'as' NAME | (','
(test|star_expr))* [','] )

It seems that if we're to support expression assignment, `as` binding
should be declined.
To be honest I feel upset because I think `expr as name` is really cool and
pythonic.

thautwarm


2018-04-13 1:35 GMT+08:00 Thautwarm Zhao <yaoxiansamma at gmail.com>:

> > Makes sense. However, couldn't you prevent that by giving with
> > priority over the binding ? As in "(with simple_cm) as value", where
> > we consider the "as" as binding operator instead of part of the with
> > statement ? Sure, you could commit suicide by parenthesis, but by
> > default it'd do exactly what the "with simple_cm as value" currently
> > does. This does require use of as instead of :=, though. (which was
> > the point I was trying to make, apologies for the confusion)
>
> Does "(with simple_cm) as value" means "with (simple_cm as value)"?
> If so, it's impossible to let the priority of "with ... as ..." over `as`
> binding.
>
> This is the grammar  of current syntax related to with statement:
>
> with_stmt: 'with' with_item (',' with_item)* ':' suite
> with_item: test ['as' expr]
>
> If `as` binding could be used in a general expression, just as
> `test` is the top of expression, an expression using `as` binding must be
> in the structure `test`.
> In other words, if you write
>
>  with expr as name:
>      # do stuff
>
> Without doubt it's equivalent to `with (expr as name)`.
>
> Or you want to completely change the grammar design of CPython :)
>
> thautwarm
>
>
>
> 2018-04-12 21:41 GMT+08:00 <python-ideas-request at python.org>:
>
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>> Today's Topics:
>>
>>    1. Re: PEP 572: Assignment Expressions (post #4) (Chris Angelico)
>>    2. Re: PEP 572: Assignment Expressions (post #4) (Chris Angelico)
>>    3. Re: PEP 572: Assignment Expressions (post #4) (Nick Coghlan)
>>    4. Re: PEP 572: Assignment Expressions (post #4) (Jacco van Dorp)
>>    5. Re: PEP 572: Assignment Expressions (post #4) (Chris Angelico)
>>
>>
>> ---------- 已转发邮件 ----------
>> From: Chris Angelico <rosuav at gmail.com>
>> To: python-ideas <python-ideas at python.org>
>> Cc:
>> Bcc:
>> Date: Thu, 12 Apr 2018 23:08:06 +1000
>> Subject: Re: [Python-ideas] PEP 572: Assignment Expressions (post #4)
>> On Thu, Apr 12, 2018 at 9:09 PM, Paul Moore <p.f.moore at gmail.com> wrote:
>> > On 11 April 2018 at 22:28, Chris Angelico <rosuav at gmail.com> wrote:
>> >> On Thu, Apr 12, 2018 at 1:22 AM, Nick Coghlan <ncoghlan at gmail.com>
>> wrote:
>> >>> This argument will be strengthened by making the examples used in the
>> >>> PEP itself more attractive, as well as proposing suitable additions to
>> >>> PEP 8, such as:
>> >>>
>> >>> 1. If either assignment statements or assignment expressions can be
>> >>> used, prefer statements
>> >>> 2. If using assignment expressions would lead to ambiguity about
>> >>> execution order, restructure to use statements instead
>> >>
>> >> Fair enough. Also adding that chained assignment expressions should
>> >> generally be avoided.
>> >
>> > Another one I think should be included (I'm a bit sad that it's not so
>> > obvious that no-one would ever even think of it, but the current
>> > discussion pretty much killed that hope for me).
>> >
>> > * Assignment expressions should never be used standalone - assignment
>> > statements should *always* be used in that case.
>>
>> That's covered by the first point. If it's a standalone statement,
>> then the statement form could be used, ergo you should prefer the
>> statement form.
>>
>> ChrisA
>>
>>
>>
>> ---------- 已转发邮件 ----------
>> From: Chris Angelico <rosuav at gmail.com>
>> To: python-ideas <python-ideas at python.org>
>> Cc:
>> Bcc:
>> Date: Thu, 12 Apr 2018 23:14:34 +1000
>> Subject: Re: [Python-ideas] PEP 572: Assignment Expressions (post #4)
>> On Thu, Apr 12, 2018 at 7:21 PM, Kirill Balunov <kirillbalunov at gmail.com>
>> wrote:
>> >
>> > I gain readability! I don't see any reason to use it in other
>> contexts...
>> > Because it makes the code unreadable and difficult to perceive while
>> giving
>> > not so much benefit.  I may be wrong, but so far I have not seen a
>> single
>> > example that at least slightly changed my mind.
>>
>> This is, in effect, your entire argument for permitting assignments
>> only in certain contexts. "I can't think of any useful reason for
>> doing this, so we shouldn't do it". But that means making the language
>> grammar more complicated (both in the technical sense of the parser's
>> definitions, and in the colloquial sense of how you'd explain Python
>> to a new programmer), because there are these magic constructs that
>> can be used anywhere in an expression, but ONLY if that expression is
>> inside an if or while statement. You lose the ability to refactor your
>> code simply to satisfy an arbitrary restriction to appease someone's
>> feeling of "it can't be useful anywhere else".
>>
>> There are basically two clean ways to do this:
>>
>> 1) Create actual syntax as part of the while statement, in the same
>> way that the 'with EXPR as NAME:' statement does. This means you
>> cannot put any additional operators after the 'as NAME' part. It's as
>> much a part of the statement's syntax as the word 'in' is in a for
>> loop.
>>
>> 2) Make this a feature of expressions in general. Then they can be
>> used anywhere that an expression can be.
>>
>> I've gone for option 2. If you want to push for option 1, go ahead,
>> but it's a nerfed solution just because you personally cannot think of
>> any good use for this.
>>
>> ChrisA
>>
>>
>>
>> ---------- 已转发邮件 ----------
>> From: Nick Coghlan <ncoghlan at gmail.com>
>> To: Chris Angelico <rosuav at gmail.com>
>> Cc: python-ideas <python-ideas at python.org>
>> Bcc:
>> Date: Thu, 12 Apr 2018 23:19:59 +1000
>> Subject: Re: [Python-ideas] PEP 572: Assignment Expressions (post #4)
>> On 12 April 2018 at 07:28, Chris Angelico <rosuav at gmail.com> wrote:
>> > On Thu, Apr 12, 2018 at 1:22 AM, Nick Coghlan <ncoghlan at gmail.com>
>> wrote:
>> >>> Frequently Raised Objections
>> >>> ============================
>> >>
>> >> There needs to be a subsection here regarding the need to call `del`
>> >> at class and module scope, just as there is for loop iteration
>> >> variables at those scopes.
>> >
>> > Hmm, I'm not sure I follow. Are you saying that this is an objection
>> > to assignment expressions, or an objection to them not being
>> > statement-local? If the latter, it's really more about "rejected
>> > alternative proposals".
>>
>> It's both - accidentally polluting class and module namespaces is an
>> argument against expression level assignments in general, and sublocal
>> namespaces aimed to eliminate that downside.
>>
>> Since feedback on the earlier versions of the PEP has moved sublocal
>> namespaces into the "rejected due to excessive conceptual complexity"
>> box, that means accidental namespace pollution comes back as a
>> downside that the PEP should mention.
>>
>> I don't think it needs to say much, just point out that they share the
>> downside of regular for loops: if you use one at class or module
>> scope, and don't want to export the name, you need to delete it
>> explicitly.
>>
>> Cheers,
>> Nick.
>>
>> --
>> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
>>
>>
>>
>> ---------- 已转发邮件 ----------
>> From: Jacco van Dorp <j.van.dorp at deonet.nl>
>> To: python-ideas <python-ideas at python.org>
>> Cc:
>> Bcc:
>> Date: Thu, 12 Apr 2018 15:31:59 +0200
>> Subject: Re: [Python-ideas] PEP 572: Assignment Expressions (post #4)
>> 2018-04-12 15:02 GMT+02:00 Nick Coghlan <ncoghlan at gmail.com>:
>> > On 12 April 2018 at 22:22, Jacco van Dorp <j.van.dorp at deonet.nl> wrote:
>> >> I've looked through PEP 343, contextlib docs (
>> >> https://docs.python.org/3/library/contextlib.html ), and I couldn't
>> >> find a single case where "with (y := f(x))" would be invalid.
>> >
>> > Consider this custom context manager:
>> >
>> >     @contextmanager
>> >     def simple_cm():
>> >         yield 42
>> >
>> > Given that example, the following code:
>> >
>> >     with cm := simple_cm() as value:
>> >         print(cm.func.__name__, value)
>> >
>> > would print "'simple_cm 42", since the assignment expression would
>> > reference the context manager itself, while the with statement binds
>> > the yielded value.
>> >
>> > Another relevant example would be `contextlib.closing`: that returns
>> > the passed in argument from __enter__, *not* self.
>> >
>> > And that's why earlier versions of PEP 572 (which used the "EXPR as
>> > NAME" spelling) just flat out prohibited top level name binding
>> > expressions in with statements: "with (expr as name):" and "with expr
>> > as name:" were far too different semantically for the only syntactic
>> > difference to be a surrounding set of parentheses.
>> >
>> > Cheers,
>> > Nick.
>>
>> Makes sense. However, couldn't you prevent that by giving with
>> priority over the binding ? As in "(with simple_cm) as value", where
>> we consider the "as" as binding operator instead of part of the with
>> statement ? Sure, you could commit suicide by parenthesis, but by
>> default it'd do exactly what the "with simple_cm as value" currently
>> does. This does require use of as instead of :=, though. (which was
>> the point I was trying to make, apologies for the confusion)
>>
>>
>>
>> ---------- 已转发邮件 ----------
>> From: Chris Angelico <rosuav at gmail.com>
>> To: python-ideas <python-ideas at python.org>
>> Cc:
>> Bcc:
>> Date: Thu, 12 Apr 2018 23:41:49 +1000
>> Subject: Re: [Python-ideas] PEP 572: Assignment Expressions (post #4)
>> On Thu, Apr 12, 2018 at 11:31 PM, Jacco van Dorp <j.van.dorp at deonet.nl>
>> wrote:
>> > 2018-04-12 15:02 GMT+02:00 Nick Coghlan <ncoghlan at gmail.com>:
>> >> On 12 April 2018 at 22:22, Jacco van Dorp <j.van.dorp at deonet.nl>
>> wrote:
>> >>> I've looked through PEP 343, contextlib docs (
>> >>> https://docs.python.org/3/library/contextlib.html ), and I couldn't
>> >>> find a single case where "with (y := f(x))" would be invalid.
>> >>
>> >> Consider this custom context manager:
>> >>
>> >>     @contextmanager
>> >>     def simple_cm():
>> >>         yield 42
>> >>
>> >> Given that example, the following code:
>> >>
>> >>     with cm := simple_cm() as value:
>> >>         print(cm.func.__name__, value)
>> >>
>> >> would print "'simple_cm 42", since the assignment expression would
>> >> reference the context manager itself, while the with statement binds
>> >> the yielded value.
>> >>
>> >> Another relevant example would be `contextlib.closing`: that returns
>> >> the passed in argument from __enter__, *not* self.
>> >>
>> >> And that's why earlier versions of PEP 572 (which used the "EXPR as
>> >> NAME" spelling) just flat out prohibited top level name binding
>> >> expressions in with statements: "with (expr as name):" and "with expr
>> >> as name:" were far too different semantically for the only syntactic
>> >> difference to be a surrounding set of parentheses.
>> >>
>> >> Cheers,
>> >> Nick.
>> >
>> > Makes sense. However, couldn't you prevent that by giving with
>> > priority over the binding ? As in "(with simple_cm) as value", where
>> > we consider the "as" as binding operator instead of part of the with
>> > statement ? Sure, you could commit suicide by parenthesis, but by
>> > default it'd do exactly what the "with simple_cm as value" currently
>> > does. This does require use of as instead of :=, though. (which was
>> > the point I was trying to make, apologies for the confusion)
>>
>> If you want this to be a generic name-binding operation, then no; most
>> objects cannot be used as context managers. You'll get an exception if
>> you try to use "with 1 as x:", for instance.
>>
>> As Nick mentioned, there are context managers that return something
>> other than 'self', and for those, "with expr as name:" has an
>> important meaning that cannot easily be captured with an assignment
>> operator.
>>
>> ChrisA
>>
>>
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