Re: Python handles globals badly.

References	(9 earlier) <55f072aa$0$1669$c3e8da3$5496439d@news.astraweb.com> <mailman.281.1441825163.8327.python-list@python.org> <55f1a8df$0$1660$c3e8da3$5496439d@news.astraweb.com> <mailman.323.1441902714.8327.python-list@python.org> <55f1c85d$0$1638$c3e8da3$5496439d@news.astraweb.com>
Date	2015-09-11 04:33 +1000
Subject	Re: Python handles globals badly.
From	Chris Angelico <rosuav@gmail.com>
Newsgroups	comp.lang.python
Message-ID	<mailman.330.1441910026.8327.python-list@python.org> (permalink)

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On Fri, Sep 11, 2015 at 4:13 AM, Steven D'Aprano <steve@pearwood.info> wrote:
> Sometimes they might be. But in general, I think it is meaningless to expect
> a imperative command to have a return result. The whole point of an
> imperative command is that it operates by side-effect.
>
> For example, what would `del x` return if it were an expression instead of a
> statement? I can think of two reasonable alternatives, but both feel a bit
> wrong to me.
>
> (1) Return True if x was successfully unbound, False if it wasn't. Except
> that raising an exception seems more Pythonic, in which case returning True
> seems redundant. It will *always* return True, unless there's an exception.
> So why bother? We only bother because there's no way to *not* return a
> result if "everything is an expression".
>
> (2) Return None, like functions do by default. But again, it only returns
> None, not because None is a meaningful thing to return, but because we
> don't actually have a way to say "it doesn't return anything".

Yes, or: (3) Return the old value that x contained, the way dict.pop()
does. That makes it a "destructive retrieval" rather than simply a
destruction operation.

> Or os.abort. The docs for that say:
>
> Help on built-in function abort in module posix:
>
> abort(...)
>     abort() -> does not return!
>
>     Abort the interpreter immediately.  This 'dumps core' or otherwise fails
>     in the hardest way possible on the hosting operating system.
>
>
> So, what would os.abort() return, if everything is an expression?

Since this is in the os module, and is thus a thin wrapper around
lower-level operations, I could imagine it returning an error code
(the way the C-level exec functions do - if they succeed, they don't
return, but if they fail, they return an error number); in Python,
it'd be best to have it either abort the process (and thus not
return), or raise an exception (and thus not return).

But abort functions are a rarity. It's not a problem to have a rule
"all functions must return something" (as Python does), and then have
functions that never actually use the normal return path:

def raise_(exc): raise exc
raise_stopiteration = iter([]).__next__
raise_typeerror = lambda: ""()

Calling one of these functions is *syntactically* an expression, but
at run time, it'll never actually get as far as producing a value. If
it's at all possible for the operation to, based on run-time
information, NOT abort, then it absolutely has to be able to return.

> Because that's the definition of an expression in this context. An
> expression is evaluated to either return a result, or raise an exception.

I'd define it more simply: An expression always produces a result.
It's possible that, during the evaluation of an expression, an
exception will be raised; if that happens, evaluation stops. Doesn't
matter whether the expression itself caused that, or if an OS-level
signal did (eg triggering KeyboardInterrupt), or if the expression was
"yield 1" and an exception got thrown in. But yes, an expression is
something that generates a result; a statement isn't.

ChrisA

Thread

Re: Python handles globals badly. tdev@freenet.de - 2015-09-02 11:47 -0700
  Re: Python handles globals badly. tdev@freenet.de - 2015-09-02 12:32 -0700
  Re: Python handles globals badly. Emile van Sebille <emile@fenx.com> - 2015-09-02 12:34 -0700
  Re: Python handles globals badly. Ian Kelly <ian.g.kelly@gmail.com> - 2015-09-02 13:38 -0600
  Re: Python handles globals badly. "Sven R. Kunze" <srkunze@mail.de> - 2015-09-02 22:08 +0200
  Re: Python handles globals badly. MRAB <python@mrabarnett.plus.com> - 2015-09-02 21:22 +0100
  Re: Python handles globals badly. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-02 22:19 +0100
  Re: Python handles globals badly. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-03 02:16 +0100
  Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-03 11:49 +1000
  Re: Python handles globals badly. Vladimir Ignatov <kmisoft@gmail.com> - 2015-09-04 20:05 -0400
  Re: Python handles globals badly. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-09-08 10:40 +0200
  Re: Python handles globals badly. Vladimir Ignatov <kmisoft@gmail.com> - 2015-09-08 07:55 -0400
  Re: Python handles globals badly. Akira Li <4kir4.1i@gmail.com> - 2015-09-08 15:35 +0300
  Re: Python handles globals badly. Mario Figueiredo <marfig@gmx.com> - 2015-09-08 14:05 +0100
  Re: Python handles globals badly. Ian Kelly <ian.g.kelly@gmail.com> - 2015-09-08 08:31 -0600
    Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-09 01:56 +1000
      Re: Python handles globals badly. Michael Torrie <torriem@gmail.com> - 2015-09-08 17:55 -0600
        Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-09 13:23 +1000
          Re: Python handles globals badly. Christian Gollwitzer <auriocus@gmx.de> - 2015-09-09 18:09 +0200
            Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-10 03:22 +1000
              Re: Python handles globals badly. Chris Angelico <rosuav@gmail.com> - 2015-09-10 03:27 +1000
              Re: Python handles globals badly. William Ray Wing <wrw@mac.com> - 2015-09-09 13:59 -0400
              Re: Python handles globals badly. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-09 20:20 +0100
                Re: Python handles globals badly. Peter Pearson <pkpearson@nowhere.invalid> - 2015-09-10 20:49 +0000
              Re: Python handles globals badly. MRAB <python@mrabarnett.plus.com> - 2015-09-09 20:22 +0100
              Re: Python handles globals badly. Chris Angelico <rosuav@gmail.com> - 2015-09-10 05:27 +1000
      Re: Python handles globals badly. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-09-09 12:42 +0200
      Re: Python handles globals badly. Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-09-09 09:33 -0400
  Re: Python handles globals badly. random832@fastmail.us - 2015-09-08 12:13 -0400
  Re: Python handles globals badly. MRAB <python@mrabarnett.plus.com> - 2015-09-08 18:41 +0100
  Re: Python handles globals badly. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-08 23:41 +0100
  Re: Python handles globals badly. MRAB <python@mrabarnett.plus.com> - 2015-09-09 00:20 +0100
  Re: Python handles globals badly. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-09 00:32 +0100
  Re: Python handles globals badly. Random832 <random832@fastmail.com> - 2015-09-08 20:02 -0400
    Re: Python handles globals badly. "Skybuck Flying" <skybuck2000@hotmail.com> - 2015-09-11 10:23 +0200
  Re: Python handles globals badly. Mario Figueiredo <marfig@gmx.com> - 2015-09-09 02:09 +0100
    Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-10 03:55 +1000
      Re: Python handles globals badly. Laura Creighton <lac@openend.se> - 2015-09-09 20:05 +0200
      Re: Python handles globals badly. Emile van Sebille <emile@fenx.com> - 2015-09-09 11:23 -0700
        Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-10 22:20 +1000
      Re: Python handles globals badly. "Sven R. Kunze" <srkunze@mail.de> - 2015-09-09 20:26 +0200
        Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-11 01:50 +1000
      Re: Python handles globals badly. random832@fastmail.us - 2015-09-09 14:59 -0400
        Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-11 01:59 +1000
          Re: Python handles globals badly. random832@fastmail.us - 2015-09-10 12:31 -0400
            Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-11 04:13 +1000
              Re: Python handles globals badly. Chris Angelico <rosuav@gmail.com> - 2015-09-11 04:33 +1000
              Re: Python handles globals badly. Jussi Piitulainen <harvesting@makes.email.invalid> - 2015-09-10 22:11 +0300
              Re: Python handles globals badly. random832@fastmail.us - 2015-09-10 16:30 -0400
          Re: Python handles globals badly. Chris Angelico <rosuav@gmail.com> - 2015-09-11 02:48 +1000
          Re: Python handles globals badly. alister <alister.nospam.ware@ntlworld.com> - 2015-09-10 19:56 +0000
          Re: Python handles globals badly. random832@fastmail.us - 2015-09-10 16:07 -0400
          Re: Python handles globals badly. Chris Angelico <rosuav@gmail.com> - 2015-09-11 11:35 +1000
          Re: Python handles globals badly. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-09-11 12:19 +0200
            Re: Python handles globals badly. Marko Rauhamaa <marko@pacujo.net> - 2015-09-11 14:59 +0300
              Re: Python handles globals badly. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-09-14 09:13 +0200
      Re: Python handles globals badly. Chris Angelico <rosuav@gmail.com> - 2015-09-10 05:00 +1000
      Re: Python handles globals badly. Laura Creighton <lac@openend.se> - 2015-09-09 21:14 +0200
      Re: Python handles globals badly. Chris Angelico <rosuav@gmail.com> - 2015-09-10 05:18 +1000
        Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-10 22:18 +1000
          Re: Python handles globals badly. jkn <jkn_gg@nicorp.f9.co.uk> - 2015-09-10 08:09 -0700
          Re: Python handles globals badly. rurpy@yahoo.com - 2015-09-10 12:04 -0700
            Re: Python handles globals badly. Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-09-10 20:33 -0400
            Re: Python handles globals badly. Gene Heskett <gheskett@wdtv.com> - 2015-09-10 22:19 -0400
            Re: Python handles globals badly. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-12 03:35 +0100
            Re: Python handles globals badly. Mario Figueiredo <marfig@gmx.com> - 2015-09-12 05:11 +0100
            Re: Python handles globals badly. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-12 05:38 +0100
            Re: Python handles globals badly. Mario Figueiredo <marfig@gmx.com> - 2015-09-12 16:52 +0100
            Re: Python handles globals badly. Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-09-12 12:24 -0400
            Re: Python handles globals badly. Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-09-12 12:29 -0400
            Re: Python handles globals badly. MRAB <python@mrabarnett.plus.com> - 2015-09-12 18:09 +0100
            Re: Python handles globals badly. Mario Figueiredo <marfig@gmx.com> - 2015-09-12 22:57 +0100
            Re: Python handles globals badly. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-12 23:05 +0100
            Re: Python handles globals badly. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-12 23:07 +0100
      Re: Python handles globals badly. "Sven R. Kunze" <srkunze@mail.de> - 2015-09-09 21:21 +0200
      Re: Python handles globals badly. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-09 20:24 +0100
      Re: Python handles globals badly. Mario Figueiredo <marfig@gmx.com> - 2015-09-09 20:57 +0100
      Re: Python handles globals badly. Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-09-09 21:15 +0100
      Re: Python handles globals badly. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-09-10 09:27 +0200
        Re: Python handles globals badly. Marko Rauhamaa <marko@pacujo.net> - 2015-09-10 10:49 +0300
      Re: Python handles globals badly. Michael Torrie <torriem@gmail.com> - 2015-09-10 08:21 -0600
      Re: Python handles globals badly. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-09-11 09:28 +0200
        Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-12 13:48 +1000
          Re: Python handles globals badly. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-09-14 10:30 +0200
            Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-16 11:13 +1000
              Re: Python handles globals badly. Steven D'Aprano <steve@pearwood.info> - 2015-09-16 11:20 +1000
                Re: Python handles globals badly. Chris Angelico <rosuav@gmail.com> - 2015-09-16 11:58 +1000
              Re: Python handles globals badly. Random832 <random832@fastmail.com> - 2015-09-15 21:54 -0400
              Re: Python handles globals badly. Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-09-16 10:51 +0200
      Re: Python handles globals badly. random832@fastmail.us - 2015-09-11 08:50 -0400
  Re: Python handles globals badly. Ben Finney <ben+python@benfinney.id.au> - 2015-09-09 11:26 +1000
  Re: Python handles globals badly. Chris Angelico <rosuav@gmail.com> - 2015-09-09 11:41 +1000
  Re: Python handles globals badly. Mario Figueiredo <marfig@gmx.com> - 2015-09-09 03:43 +0100
  Re: Python handles globals badly. Chris Angelico <rosuav@gmail.com> - 2015-09-09 13:03 +1000
  Re: Python handles globals badly. Ben Finney <ben+python@benfinney.id.au> - 2015-09-09 15:53 +1000

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