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Re: code review

Date 2012-07-01 01:50 +0200
From Thomas Jollans <t@jollybox.de>
Subject Re: code review
References (10 earlier) <87wr2oecf6.fsf@dpt-info.u-strasbg.fr> <4FEF7117.7000109@jollybox.de> <jsns6l$7c0$1@dough.gmane.org> <4FEF7826.40900@jollybox.de> <CAPTjJmos+Xg+m3EoMbNKJo=268j1=2waKBcM1FMFNJUKTRx6+A@mail.gmail.com>
Newsgroups comp.lang.python
Message-ID <mailman.1665.1341100210.4697.python-list@python.org> (permalink)

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On 07/01/2012 01:25 AM, Chris Angelico wrote:
> On Sun, Jul 1, 2012 at 8:05 AM, Thomas Jollans <t@jollybox.de> wrote:
>> Yes. My sole point, really, is that "normally", one would expect these
>> two expressions to be equivalent:
>>
>> a < b < c
>> (a < b) < c
>>
>> This is clearly not true.
> 
> Python has quite a few things like that, actually. The most noticeable
> for C programmers is:
> 
> a = b = c = d = e = 0
> 
> which in C is identical to
> 
> a = (b = (c = (d = (e = 0))))
> 
> because assignment is an expression, but in Python is equivalent to
> nothing because assignment is simply allowed to do multiple. Downside:
> *Only* simple assignment can be chained. Augmented assignment cannot:
> 
>>>> a+=10        # That's fine.
>>>> a+=b+=10
>   File "<stdin>", line 1
>     a+=b+=10
>          ^
> SyntaxError: invalid syntax
>>>> a=b+=10
>   File "<stdin>", line 1
>     a=b+=10
>         ^
> SyntaxError: invalid syntax
>>>> a+=b=10
>   File "<stdin>", line 1
>     a+=b=10
>         ^
> SyntaxError: invalid syntax
> 
> 
> In C, these are all well-defined and valid, and are evaluated
> right-to-left, and do what you would expect. (And yes, it's handy at
> times to do this sort of thing.)
> 
> So it's not a special case for the comparison operators; it's a more
> general case that Python handles certain chains of operators as single
> entities, rather than breaking everything down into "OPERAND OPERATOR
> OPERAND" and evaluating one at a time. Is it better than/worse than C?
> Not really. It's a design choice and we code within it. (My personal
> preference is for the C style, as it makes for a more expressive
> language with less mental glitching, but as I say, that's personal
> preference.)

Nicely put. Of course it's not catastrophic, it's just a feature of
Python that I'm not particularly fond of.

Another operator with special chaining behaviour that occurred to me is
the tuple-building "," operator. This is a particularly interesting one
since the "," symbol is also used in other contexts where it is not an
operator, and the symbol can be considered an operator in the way it can
be in Python only in very few (if any) other programming languages.

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code review "Littlefield, Tyler" <tyler@tysdomain.com> - 2012-06-28 20:57 -0600
  Re: code review alex23 <wuwei23@gmail.com> - 2012-06-28 20:58 -0700
    Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-06-29 07:31 +0000
      Re: code review Chris Angelico <rosuav@gmail.com> - 2012-06-29 17:42 +1000
      Re: code review "Littlefield, Tyler" <tyler@tysdomain.com> - 2012-06-29 09:03 -0600
        Re: code review Alister <alister.ware@ntlworld.com> - 2012-06-29 19:41 +0000
          Re: code review MRAB <python@mrabarnett.plus.com> - 2012-06-29 21:09 +0100
          Re: code review "Martin P. Hellwig" <martin.hellwig@gmail.com> - 2012-06-29 13:27 -0700
            Re: code review Alister <alister.ware@ntlworld.com> - 2012-06-29 20:43 +0000
              Re: code review Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-06-29 19:02 -0400
              Re: code review Terry Reedy <tjreedy@udel.edu> - 2012-06-29 23:02 -0400
          Re: code review "Littlefield, Tyler" <tyler@tysdomain.com> - 2012-06-29 14:49 -0600
            Re: code review Alister <alister.ware@ntlworld.com> - 2012-06-30 09:31 +0000
              Re: code review Alister <alister.ware@ntlworld.com> - 2012-06-30 09:36 +0000
          Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-06-30 02:28 +0000
            Re: code review Alister <alister.ware@ntlworld.com> - 2012-06-30 09:22 +0000
          Re: code review Terry Reedy <tjreedy@udel.edu> - 2012-06-29 23:00 -0400
        Re: code review Alister <alister.ware@ntlworld.com> - 2012-06-30 10:04 +0000
          Re: code review Peter Otten <__peter__@web.de> - 2012-06-30 12:29 +0200
            Re: code review Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2012-06-30 20:39 +0200
              Re: code review Thomas Jollans <t@jollybox.de> - 2012-06-30 21:38 +0200
                Re: code review Alister <alister.ware@ntlworld.com> - 2012-06-30 20:30 +0000
                Re: code review Thomas Jollans <t@jollybox.de> - 2012-06-30 22:50 +0200
                Re: code review Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2012-06-30 23:07 +0200
                Re: code review Thomas Jollans <t@jollybox.de> - 2012-06-30 23:35 +0200
                Re: code review Terry Reedy <tjreedy@udel.edu> - 2012-06-30 17:47 -0400
                Re: code review Thomas Jollans <t@jollybox.de> - 2012-07-01 00:05 +0200
                Re: code review Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2012-07-01 01:03 +0200
                Re: code review Ben Finney <ben+python@benfinney.id.au> - 2012-07-01 10:08 +1000
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-01 10:37 +1000
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-01 03:23 +0000
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-01 13:48 +1000
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-01 06:54 +0000
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-01 16:59 +1000
                Re: code review Terry Reedy <tjreedy@udel.edu> - 2012-07-01 05:55 -0400
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-02 01:26 +0000
                Re: code review Albert van der Horst <albert@spenarnc.xs4all.nl> - 2012-07-13 12:30 +0000
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-13 15:04 +0000
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-14 01:36 +1000
                Re: code review rusi <rustompmody@gmail.com> - 2012-07-13 09:24 -0700
                Re: code review Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-07-13 16:39 -0400
                Re: code review Duncan Booth <duncan.booth@invalid.invalid> - 2012-07-16 10:43 +0000
                Re: code review Ben Finney <ben+python@benfinney.id.au> - 2012-07-16 21:34 +1000
                Re: code review Albert van der Horst <albert@spenarnc.xs4all.nl> - 2012-07-17 10:54 +0000
                Re: code review Terry Reedy <tjreedy@udel.edu> - 2012-07-13 19:09 -0400
                Re: code review Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-14 03:26 -0600
                Re: code review Terry Reedy <tjreedy@udel.edu> - 2012-07-14 16:42 -0400
                Re: code review rusi <rustompmody@gmail.com> - 2012-06-30 21:07 -0700
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-01 14:20 +1000
                Re: code review Ben Finney <ben+python@benfinney.id.au> - 2012-07-01 17:28 +1000
                Re: code review Thomas Jollans <t@jollybox.de> - 2012-07-01 09:46 +0200
                Re: code review HoneyMonster <nobody@someplace.invalid> - 2012-07-01 20:53 +0000
                Re: code review Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-07-01 05:18 -0400
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-02 00:41 +0000
                Re: code review Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-07-01 21:40 -0400
                Re: code review Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-07-01 13:41 -0400
                Re: code review John O'Hagan <research@johnohagan.com> - 2012-07-02 14:43 +1000
                Re: Re: code review Evan Driscoll <driscoll@cs.wisc.edu> - 2012-06-30 23:45 -0500
                Re: Re: code review Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2012-07-01 08:57 +0200
                Re: code review Alister <alister.ware@ntlworld.com> - 2012-07-01 09:54 +0000
                Re: Re: code review Evan Driscoll <driscoll@cs.wisc.edu> - 2012-07-01 10:48 -0500
                Re: Re: code review lars van gemerden <lars@rational-it.com> - 2012-07-06 04:22 -0700
                Re: Re: code review lars van gemerden <lars@rational-it.com> - 2012-07-06 04:22 -0700
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-06 13:58 +0000
                Re: code review Roy Smith <roy@panix.com> - 2012-07-13 08:32 -0700
                Re: code review Evan Driscoll <driscoll@cs.wisc.edu> - 2012-06-30 23:57 -0500
                Re: code review Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2012-07-01 09:04 +0200
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-01 02:06 +0000
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-01 12:20 +1000
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-01 04:17 +0000
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-01 14:23 +1000
                Re: code review Steven D'Aprano <steve+usenet@pearwood.info> - 2012-07-01 06:27 +0000
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-01 16:33 +1000
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-02 01:28 +0000
                Re: code review Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-07-01 21:50 -0400
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-02 07:29 +0000
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-02 12:04 +1000
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-02 08:11 +0000
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-02 18:20 +1000
                Re: code review Rick Johnson <rantingrickjohnson@gmail.com> - 2012-07-02 08:57 -0700
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-03 02:42 +1000
                Re: code review Rick Johnson <rantingrickjohnson@gmail.com> - 2012-07-02 11:22 -0700
                Re: code review Thomas Jollans <t@jollybox.de> - 2012-07-02 21:06 +0200
                Re: code review Rick Johnson <rantingrickjohnson@gmail.com> - 2012-07-02 12:35 -0700
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-03 07:57 +1000
                Re: code review Neil Cerutti <neilc@norwich.edu> - 2012-07-03 12:19 +0000
                Re: code review Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-07-02 01:20 -0400
                Re: code review Thomas Jollans <t@jollybox.de> - 2012-07-02 16:41 +0200
                Re: code review Terry Reedy <tjreedy@udel.edu> - 2012-07-02 11:33 -0400
                Re: code review Thomas Jollans <t@jollybox.de> - 2012-07-01 09:35 +0200
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-02 00:43 +0000
                Re: code review Thomas Jollans <t@jollybox.de> - 2012-07-02 16:26 +0200
                Re: code review Rick Johnson <rantingrickjohnson@gmail.com> - 2012-07-02 08:16 -0700
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-03 02:55 +1000
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-03 00:57 +0000
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-03 11:22 +1000
                Re: code review John O'Hagan <research@johnohagan.com> - 2012-07-03 12:25 +1000
                Re: code review Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-03 04:11 +0000
                Re: code review Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-07-03 02:09 -0400
                Re: code review Roy Smith <roy@panix.com> - 2012-07-03 08:33 -0400
                Re: code review Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-07-03 16:53 +0100
                Re: code review Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-07-03 17:32 -0400
                Re: code review rusi <rustompmody@gmail.com> - 2012-07-02 22:10 -0700
                Re: code review Ben Finney <ben+python@benfinney.id.au> - 2012-07-03 15:46 +1000
                Re: code review John O'Hagan <research@johnohagan.com> - 2012-07-04 00:59 +1000
                Re: code review Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-07-03 16:50 +0100
                Re: code review Paul Rudin <paul.nospam@rudin.co.uk> - 2012-07-04 10:29 +0100
                Re: code review Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-07-04 17:25 +0100
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-04 01:53 +1000
                Re: code review Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-07-03 17:05 +0100
                Re: code review Dave Angel <d@davea.name> - 2012-07-03 16:13 -0400
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-04 07:54 +1000
                Re: code review Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-07-04 09:28 +0100
                Re: code review rusi <rustompmody@gmail.com> - 2012-06-30 19:37 -0700
                Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-01 09:25 +1000
                Re: code review Thomas Jollans <t@jollybox.de> - 2012-07-01 01:50 +0200
                Re: code review "Martin P. Hellwig" <martin.hellwig@gmail.com> - 2012-06-30 14:48 -0700
                Re: code review Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-02 13:16 -0600
            Re: code review Alister <alister.ware@ntlworld.com> - 2012-06-30 20:25 +0000
          Re: code review Kushal Kumaran <kushal.kumaran+python@gmail.com> - 2012-07-03 23:23 +0530
            Re: code review John Gordon <gordon@panix.com> - 2012-07-03 18:18 +0000
              Re: code review Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-03 12:27 -0600
              Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-04 07:51 +1000
          Re: code review Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-03 12:19 -0600
          Re: code review kushal.kumaran+python@gmail.com - 2012-07-04 08:27 +0530
          Re: code review Chris Angelico <rosuav@gmail.com> - 2012-07-04 13:53 +1000
          Re: code review Simon Cropper <simoncropper@fossworkflowguides.com> - 2012-07-04 14:55 +1000
          Re: code review "Littlefield, Tyler" <tyler@tysdomain.com> - 2012-07-03 23:39 -0600
            Re: code review alex23 <wuwei23@gmail.com> - 2012-07-03 23:17 -0700
              Re: code review rusi <rustompmody@gmail.com> - 2012-07-04 00:05 -0700
          Apology for OT posts (was: code review) John O'Hagan <research@johnohagan.com> - 2012-07-06 12:06 +1000
          Re: Apology for OT posts Simon Cropper <simoncropper@fossworkflowguides.com> - 2012-07-06 15:30 +1000
          Re: Apology for OT posts Chris Angelico <rosuav@gmail.com> - 2012-07-06 17:45 +1000
          Re: Apology for OT posts Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-07-06 10:37 +0100

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