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Re: Case Statements

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From Chris Angelico <rosuav@gmail.com>
Newsgroups comp.lang.python
Subject Re: Case Statements
Date Thu, 17 Mar 2016 17:48:58 +1100
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On Thu, Mar 17, 2016 at 5:29 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> I don't think that property is a similar situation. I think what happens
> here is that the first call to property sets:
>
> # @property def x...
> x = property(x)
>
> Then the second decorator does:
>
> # @x.setter def x...
> x = x.setter(x)
>
> which replaces x with a brand new property object.

Okay. Let's try this.

>>> class Demo:
...     @property
...     def x(self):
...         print("Getting x")
...         return 42
...     @x.setter
...     def x(self, value):
...         print("Setting x to", value)
...
>>> d = Demo()
>>> d.x
Getting x
42
>>> d.x = 1
Setting x to 1


Decorators work. Now let's try NOT using decorators.

>>> class Demo:
...     def x(self):
...         print("Getting x")
...         return 42
...     x = property(x)
...     def x(self, value):
...         print("Setting x to", value)
...     x = x.setter(x)
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 8, in Demo
AttributeError: 'function' object has no attribute 'setter'


Since the 'def' line bound the undecorated function to the name 'x',
the decoration underneath *fails*. The only way this could work would
be with a temporary name for either the new function or the decorator:

>>> class Demo:
...     def x(self):
...         print("Getting x")
...         return 42
...     x = property(x)
...     decorator = x.setter
...     def x(self, value):
...         print("Setting x to", value)
...     x = decorator(x)
...     del decorator
...
>>> d = Demo()
>>> d.x
Getting x
42
>>> d.x = 1
Setting x to 1

This is how CPython implements this (it evaluates the decoration
expressions first, leaving them on the stack, then creates the
function, then calls the decorators), but I don't see anywhere that
this is guaranteed in the docs either - the only way I know this is
current behaviour is from trying it (with dis.dis). The documented
equivalence simply doesn't work.

Note, by the way, that I am not in any way criticising the *behaviour*
here. I don't think anything needs to be changed as regards
functionality. And it's not even that big an issue as regards
documentation. It's mainly just an esoteric curiosity.

ChrisA

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Case Statements jj0gen0info@gmail.com - 2016-03-15 13:46 -0700
  Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-15 23:11 +0000
  Re: Case Statements jj0gen0info@gmail.com - 2016-03-15 16:47 -0700
    Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-15 23:58 +0000
    Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 00:51 +0000
      Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 01:05 +0000
  Re: Case Statements jj0gen0info@gmail.com - 2016-03-15 18:55 -0700
    Re: Case Statements "Mario R. Osorio" <nimbiotics@gmail.com> - 2016-03-15 21:06 -0700
      Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 10:34 +0000
        Re: Case Statements Steven D'Aprano <steve@pearwood.info> - 2016-03-16 23:56 +1100
    Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 04:26 +0000
      Re: Case Statements Christian Gollwitzer <auriocus@gmx.de> - 2016-03-16 09:13 +0100
        Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 08:47 +0000
        Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 11:16 +0200
        Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-16 10:35 +0100
        Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 09:51 +0000
          Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 19:41 +0000
            Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 22:30 +0000
        Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-16 11:52 +0100
        Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 11:07 +0000
          Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 11:16 +0000
            Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 11:33 +0000
            Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 14:21 +0200
              Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 12:53 +0000
                Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 16:31 +0200
                Re: Case Statements BartC <bc@freeuk.com> - 2016-03-16 15:00 +0000
                Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 19:07 +0200
              Re: Case Statements Rustom Mody <rustompmody@gmail.com> - 2016-03-20 01:01 -0700
                Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-20 08:29 +0000
        Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-16 14:38 +0100
        Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 14:02 +0000
        Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 21:27 +0200
          Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-17 09:22 +0100
            Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-17 10:57 +0200
              Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-17 10:12 +0100
        Re: Case Statements Steven D'Aprano <steve@pearwood.info> - 2016-03-17 11:19 +1100
          Re: Case Statements Steven D'Aprano <steve@pearwood.info> - 2016-03-17 12:54 +1100
            Re: Case Statements Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2016-03-17 18:45 +1300
              Re: Case Statements Chris Angelico <rosuav@gmail.com> - 2016-03-17 17:30 +1100
                Re: Case Statements Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2016-03-18 18:59 +1300
              Re: Case Statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-03-17 17:29 +1100
                Re: Case Statements Chris Angelico <rosuav@gmail.com> - 2016-03-17 17:48 +1100
                Re: Case Statements Steven D'Aprano <steve@pearwood.info> - 2016-03-17 21:53 +1100
                Re: Case Statements Chris Angelico <rosuav@gmail.com> - 2016-03-17 22:49 +1100
            Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-17 10:23 +0100
            Re: Case Statements Chris Angelico <rosuav@gmail.com> - 2016-03-17 20:55 +1100
          Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-17 09:36 +0100
        Re: Case Statements Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-17 09:38 +0100
  Re: Case Statements l0r0m0a0i0l@gmail.com - 2016-03-16 06:15 -0700
    Re: Case Statements Steven D'Aprano <steve@pearwood.info> - 2016-03-17 00:27 +1100
      Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 14:00 +0000
    Re: Case Statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-16 13:59 +0000
  Re: Case Statements l0r0m0a0i0l@gmail.com - 2016-03-16 07:21 -0700
    Re: Case Statements Marko Rauhamaa <marko@pacujo.net> - 2016-03-16 16:33 +0200

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