Re: singleton ... again

From	Ned Batchelder <ned@nedbatchelder.com>
Subject	Re: singleton ... again
Date	2014-02-13 06:50 -0500
References	(1 earlier) <mailman.6728.1392183929.18130.python-list@python.org> <roy-11BAB5.00555912022014@news.panix.com> <bm0v9iFpuquU1@mid.individual.net> <mailman.6750.1392199807.18130.python-list@python.org> <m2a9dvmlos.fsf@cochabamba.vanoostrum.org>
Newsgroups	comp.lang.python
Message-ID	<mailman.6831.1392292254.18130.python-list@python.org> (permalink)

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On 2/13/14 4:00 AM, Piet van Oostrum wrote:
> Ben Finney <ben+python@benfinney.id.au> writes:
>
>> Gregory Ewing <greg.ewing@canterbury.ac.nz> writes:
>>
>>> Roy Smith wrote:
>>>> It looks to me like he's trying to implement a classic Gang of Four
>>>> singleton pattern.
>>>
>>> Which I've never really seen the point of in Python, or any other
>>> language for that matter. Just create one instance of the class during
>>> initialisation, put it in a global somewhere, and use it thereafter.
>>
>> Make that “somewhere” a module namespace, and you effectively have a
>> Singleton for all practical purposes. So yes, I see the point of it; but
>> we already have it built in :-)
>
> There is a use case for a singleton class: when creating the singleton
> object takes considerable resources and you don't need it always in your
> program.
>

I still don't see it.  To convince me that a singleton class makes 
sense, you'd have to explain why by virtue of the class's very nature, 
it never makes sense for there ever to be more than one of them.

Your example is an expensive-to-create object.  Why does that mean I 
might not want two of them?  I can see how it makes sense to have a 
factory function, which will make one only when asked, and will hold 
onto that object for the next time it's needed.

But that's different than a class which pretends to make instances but 
actually always returns the same instance.

-- 
Ned Batchelder, http://nedbatchelder.com

Thread

singleton ... again Asaf Las <roegltd@gmail.com> - 2014-02-11 20:34 -0800
  Re: singleton ... again Asaf Las <roegltd@gmail.com> - 2014-02-11 21:15 -0800
  Re:singleton ... again Dave Angel <davea@davea.name> - 2014-02-12 00:48 -0500
    Re: singleton ... again Roy Smith <roy@panix.com> - 2014-02-12 00:55 -0500
      Re: singleton ... again Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-12 23:04 +1300
        Re: singleton ... again Ben Finney <ben+python@benfinney.id.au> - 2014-02-12 21:09 +1100
          Re: singleton ... again Roy Smith <roy@panix.com> - 2014-02-12 08:40 -0500
          Re: singleton ... again Piet van Oostrum <piet@vanoostrum.org> - 2014-02-13 10:00 +0100
            Re: singleton ... again Ned Batchelder <ned@nedbatchelder.com> - 2014-02-13 06:50 -0500
            Re: singleton ... again Chris Angelico <rosuav@gmail.com> - 2014-02-13 22:57 +1100
              Re: singleton ... again Roy Smith <roy@panix.com> - 2014-02-13 10:24 -0500
                Re: singleton ... again Chris Angelico <rosuav@gmail.com> - 2014-02-14 07:03 +1100
                Re: singleton ... again Robert Kern <robert.kern@gmail.com> - 2014-02-13 21:13 +0000
                Re: singleton ... again Chris Angelico <rosuav@gmail.com> - 2014-02-14 08:27 +1100
            Re: singleton ... again Ethan Furman <ethan@stoneleaf.us> - 2014-02-13 08:57 -0800
              Re: singleton ... again Roy Smith <roy@panix.com> - 2014-02-13 12:57 -0500
                Re: singleton ... again Ethan Furman <ethan@stoneleaf.us> - 2014-02-13 10:31 -0800
                Re: singleton ... again Roy Smith <roy@panix.com> - 2014-02-13 14:03 -0500
                Re: singleton ... again Chris Angelico <rosuav@gmail.com> - 2014-02-14 06:50 +1100
                Re: singleton ... again Grant Edwards <invalid@invalid.invalid> - 2014-02-13 22:28 +0000
              Re: singleton ... again Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-14 18:21 +1300
            Re: singleton ... again Tim Delaney <timothy.c.delaney@gmail.com> - 2014-02-14 06:16 +1100
        Re: singleton ... again Steven D'Aprano <steve@pearwood.info> - 2014-02-13 02:58 +0000
          Re: singleton ... again Ben Finney <ben+python@benfinney.id.au> - 2014-02-13 14:07 +1100
            Re: singleton ... again Steven D'Aprano <steve@pearwood.info> - 2014-02-13 04:24 +0000
              Re: singleton ... again Chris Angelico <rosuav@gmail.com> - 2014-02-13 15:33 +1100
              Re: singleton ... again Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-14 18:15 +1300
          Re: singleton ... again Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-14 18:06 +1300
    Re: singleton ... again Asaf Las <roegltd@gmail.com> - 2014-02-12 09:50 -0800
      Re: singleton ... again Asaf Las <roegltd@gmail.com> - 2014-02-12 09:56 -0800
        Re: singleton ... again Asaf Las <roegltd@gmail.com> - 2014-02-12 09:58 -0800
        Re: singleton ... again Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2014-02-13 10:57 +1300
          Re: singleton ... again Asaf Las <roegltd@gmail.com> - 2014-02-12 17:38 -0800
      Re: singleton ... again Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-02-12 18:57 +0000
        Re: singleton ... again Asaf Las <roegltd@gmail.com> - 2014-02-12 17:02 -0800
      Re: singleton ... again Ned Batchelder <ned@nedbatchelder.com> - 2014-02-12 16:34 -0500
        Re: singleton ... again Asaf Las <roegltd@gmail.com> - 2014-02-12 17:34 -0800
      Re: singleton ... again Tim Delaney <timothy.c.delaney@gmail.com> - 2014-02-13 10:08 +1100
      Re: singleton ... again Roy Smith <roy@panix.com> - 2014-02-12 20:57 -0500
  Re: singleton ... again Michael Torrie <torriem@gmail.com> - 2014-02-12 15:05 -0700

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