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Re: What is considered an "advanced" topic in Python?

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Date Fri, 29 May 2015 14:39:44 -0700
From Ethan Furman <ethan@stoneleaf.us>
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On 05/29/2015 02:06 PM, sohcahtoa82@gmail.com wrote:
> On Friday, May 29, 2015 at 10:18:29 AM UTC-7, Ethan Furman wrote:
>>
>> Metaclasses change the way a class behaves.
>>
>> For example, the new (in 3.4) Enum class uses a metaclass.
>>
>>     class SomeEnum(Enum):
>>        first = 1
>>        second = 2
>>        third = 3
>>
>> The metaclass changes normal class behavior to:
>>
>>     - support iterating: list(SomeEnum) --> [SomeEnum.first, SomeEnum.second, SomeEnum.third]
>>     - support a length:  len(SomeEnum) --> 3
>>     - not allow new instances to be created:  --> SomeEnum(1) is SomeEnum(1)  # True
>>
>> --
>> ~Ethan~
>
> Regarding the first two, you can implement __iter__ and __len__ functions to create that functionality, though those functions would operate on an instance of the class, not the class itself.

Hence the need for a metaclass, as the point is to operate on the class, not the instance.

> As for the third, can't you override the __new__ function to make attempts to create a new instance just return a previously created instance?

Yes.  In the case of Enum, however, it takes any user-defined __new__, which is needed for creating the original instances, and replaces it with a __new__ that only returns the already defined ones.

--
~Ethan~

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What is considered an "advanced" topic in Python? Mike Driscoll <kyosohma@gmail.com> - 2015-05-29 09:01 -0700
  Re: What is considered an "advanced" topic in Python? Joel Goldstick <joel.goldstick@gmail.com> - 2015-05-29 12:08 -0400
    Re: What is considered an "advanced" topic in Python? Mike Driscoll <kyosohma@gmail.com> - 2015-05-29 09:55 -0700
  Re: What is considered an "advanced" topic in Python? Chris Angelico <rosuav@gmail.com> - 2015-05-30 02:09 +1000
    Re: What is considered an "advanced" topic in Python? Mike Driscoll <kyosohma@gmail.com> - 2015-05-29 09:57 -0700
      Re: What is considered an "advanced" topic in Python? Chris Angelico <rosuav@gmail.com> - 2015-05-30 03:08 +1000
      Re: What is considered an "advanced" topic in Python? random832@fastmail.us - 2015-05-31 23:18 -0400
      Re: What is considered an "advanced" topic in Python? Chris Angelico <rosuav@gmail.com> - 2015-06-01 13:43 +1000
      Re: What is considered an "advanced" topic in Python? Laura Creighton <lac@openend.se> - 2015-06-01 09:58 +0200
        Re: What is considered an "advanced" topic in Python? Marko Rauhamaa <marko@pacujo.net> - 2015-06-01 12:36 +0300
          Zero [was Re: What is considered an "advanced" topic in Python?] Steven D'Aprano <steve@pearwood.info> - 2015-06-01 22:07 +1000
            Re: Zero [was Re: What is considered an "advanced" topic in Python?] Laura Creighton <lac@openend.se> - 2015-06-01 14:52 +0200
            Re: Zero [was Re: What is considered an "advanced" topic in Python?] Dave Farrance <DaveFarrance@OMiTTHiSyahooANDTHiS.co.uk> - 2015-06-01 13:56 +0100
              Re: Zero [was Re: What is considered an "advanced" topic in Python?] Skip Montanaro <skip.montanaro@gmail.com> - 2015-06-01 08:14 -0500
                Re: Zero [was Re: What is considered an "advanced" topic in Python?] Dave Farrance <df@see.replyto.invalid> - 2015-06-01 15:39 +0100
              Re: Zero [was Re: What is considered an "advanced" topic in Python?] Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-06-01 18:35 +0100
              Re: Zero [was Re: What is considered an "advanced" topic in Python?] Terry Reedy <tjreedy@udel.edu> - 2015-06-01 17:32 -0400
            Re: Zero [was Re: What is considered an "advanced" topic in Python?] Marko Rauhamaa <marko@pacujo.net> - 2015-06-01 16:18 +0300
              Re: Zero [was Re: What is considered an "advanced" topic in Python?] random832@fastmail.us - 2015-06-01 09:32 -0400
        Re: What is considered an "advanced" topic in Python? Dave Farrance <DaveFarrance@OMiTTHiSyahooANDTHiS.co.uk> - 2015-06-01 11:44 +0100
          Re: What is considered an "advanced" topic in Python? Marko Rauhamaa <marko@pacujo.net> - 2015-06-01 13:54 +0300
        Re: What is considered an "advanced" topic in Python? Rustom Mody <rustompmody@gmail.com> - 2015-06-01 22:33 -0700
      Re: What is considered an "advanced" topic in Python? Chris Angelico <rosuav@gmail.com> - 2015-06-01 19:45 +1000
      Re: What is considered an "advanced" topic in Python? Laura Creighton <lac@openend.se> - 2015-06-01 12:28 +0200
        Re: What is considered an "advanced" topic in Python? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-06-01 14:22 +0100
      Re: What is considered an "advanced" topic in Python? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-06-01 11:34 +0100
      Re: What is considered an "advanced" topic in Python? Chris Angelico <rosuav@gmail.com> - 2015-06-01 20:36 +1000
      Re: What is considered an "advanced" topic in Python? Laura Creighton <lac@openend.se> - 2015-06-01 13:00 +0200
      Re: What is considered an "advanced" topic in Python? Laura Creighton <lac@openend.se> - 2015-06-01 13:24 +0200
        Re: What is considered an "advanced" topic in Python? Marko Rauhamaa <marko@pacujo.net> - 2015-06-01 14:57 +0300
          Re: What is considered an "advanced" topic in Python? BartC <bc@freeuk.com> - 2015-06-01 13:27 +0100
          Re: What is considered an "advanced" topic in Python? MRAB <python@mrabarnett.plus.com> - 2015-06-01 13:27 +0100
          Re: What is considered an "advanced" topic in Python? Laura Creighton <lac@openend.se> - 2015-06-01 17:07 +0200
            Re: What is considered an "advanced" topic in Python? alister <alister.nospam.ware@ntlworld.com> - 2015-06-01 15:26 +0000
              Re: What is considered an "advanced" topic in Python? Laura Creighton <lac@openend.se> - 2015-06-01 17:51 +0200
              Re: What is considered an "advanced" topic in Python? MRAB <python@mrabarnett.plus.com> - 2015-06-01 18:06 +0100
                Re: What is considered an "advanced" topic in Python? Marko Rauhamaa <marko@pacujo.net> - 2015-06-01 20:14 +0300
            Re: What is considered an "advanced" topic in Python? BartC <bc@freeuk.com> - 2015-06-01 16:42 +0100
              Re: What is considered an "advanced" topic in Python? Grant Edwards <invalid@invalid.invalid> - 2015-06-01 17:02 +0000
                Re: What is considered an "advanced" topic in Python? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-06-01 18:45 +0100
                Re: What is considered an "advanced" topic in Python? Grant Edwards <invalid@invalid.invalid> - 2015-06-01 18:23 +0000
        Re: What is considered an "advanced" topic in Python? BartC <bc@freeuk.com> - 2015-06-01 13:24 +0100
      Re: What is considered an "advanced" topic in Python? Chris Angelico <rosuav@gmail.com> - 2015-06-01 23:52 +1000
        Re: What is considered an "advanced" topic in Python? BartC <bc@freeuk.com> - 2015-06-01 16:17 +0100
          Re: What is considered an "advanced" topic in Python? Chris Angelico <rosuav@gmail.com> - 2015-06-02 02:10 +1000
            Re: What is considered an "advanced" topic in Python? Rustom Mody <rustompmody@gmail.com> - 2015-06-01 21:46 -0700
  Re: What is considered an "advanced" topic in Python? Skip Montanaro <skip.montanaro@gmail.com> - 2015-05-29 11:39 -0500
    Re: What is considered an "advanced" topic in Python? Mike Driscoll <kyosohma@gmail.com> - 2015-05-29 09:58 -0700
  Re: What is considered an "advanced" topic in Python? Chris Angelico <rosuav@gmail.com> - 2015-05-30 02:50 +1000
  Re: What is considered an "advanced" topic in Python? Todd <toddrjen@gmail.com> - 2015-05-29 19:02 +0200
  Re: What is considered an "advanced" topic in Python? sohcahtoa82@gmail.com - 2015-05-29 10:03 -0700
    Re: What is considered an "advanced" topic in Python? Ethan Furman <ethan@stoneleaf.us> - 2015-05-29 10:17 -0700
      Re: What is considered an "advanced" topic in Python? sohcahtoa82@gmail.com - 2015-05-29 14:06 -0700
        Re: What is considered an "advanced" topic in Python? Jason Swails <jason.swails@gmail.com> - 2015-05-29 17:28 -0400
        Re: What is considered an "advanced" topic in Python? Ethan Furman <ethan@stoneleaf.us> - 2015-05-29 14:39 -0700
        Re: What is considered an "advanced" topic in Python? Ethan Furman <ethan@stoneleaf.us> - 2015-05-29 14:44 -0700
    Re: What is considered an "advanced" topic in Python? Marko Rauhamaa <marko@pacujo.net> - 2015-05-29 20:24 +0300
  Re: What is considered an "advanced" topic in Python? Todd <toddrjen@gmail.com> - 2015-05-29 19:03 +0200
  Re: What is considered an "advanced" topic in Python? Steven D'Aprano <steve@pearwood.info> - 2015-05-30 03:55 +1000
    Re: What is considered an "advanced" topic in Python? Mike Driscoll <kyosohma@gmail.com> - 2015-05-29 13:38 -0700
  Re: What is considered an "advanced" topic in Python? Sturla Molden <sturla.molden@gmail.com> - 2015-05-30 12:15 +0000
  Re: What is considered an "advanced" topic in Python? jonathon <jonathon.blake@gmail.com> - 2015-05-30 19:32 +0000
  Re: What is considered an "advanced" topic in Python? Chris Angelico <rosuav@gmail.com> - 2015-05-31 08:24 +1000
  Re: What is considered an "advanced" topic in Python? "C.D. Reimer" <chris@cdreimer.com> - 2015-05-30 18:28 -0700
  Re: What is considered an "advanced" topic in Python? Rustom Mody <rustompmody@gmail.com> - 2015-05-30 20:30 -0700
    Re: What is considered an "advanced" topic in Python? Chris Angelico <rosuav@gmail.com> - 2015-05-31 14:25 +1000
      Re: What is considered an "advanced" topic in Python? Rustom Mody <rustompmody@gmail.com> - 2015-05-30 21:46 -0700
        Re: What is considered an "advanced" topic in Python? Chris Angelico <rosuav@gmail.com> - 2015-05-31 14:58 +1000
          Re: What is considered an "advanced" topic in Python? Rustom Mody <rustompmody@gmail.com> - 2015-05-30 22:18 -0700
  Re: What is considered an "advanced" topic in Python? Gene Heskett <gheskett@wdtv.com> - 2015-06-01 09:49 -0400
  Re: What is considered an "advanced" topic in Python? Denis McMahon <denismfmcmahon@gmail.com> - 2015-06-01 15:30 +0000
  Re: What is considered an "advanced" topic in Python? Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2015-06-01 20:33 -0400
  Re: What is considered an "advanced" topic in Python? Gene Heskett <gheskett@wdtv.com> - 2015-06-01 23:30 -0400

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