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Re: Interesting list() un-optimization

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On 3/10/2013 9:05 AM, Roy Smith wrote:
> In article <roy-572C99.22201106032013@70-1-84-166.pools.spcsdns.net>,
>   Roy Smith <roy@panix.com> wrote:
>
>> The problem is, QuerySets have a __len__() method.  Calling it is a lot
>> faster than iterating over the whole query set and counting the items,
>> but it does result in an additional database query, which is a lot
>> slower than the list resizing!  Writing the code as a list comprehension
>> prevents list() from trying to optimize when it shouldn't!
>
> Hmmm, I think I've found a good solution.
>
> It turns out, we don't actually use QuerySet in our models.  We've
> defined our own QuerySet subclass which adds a few convenience methods.
> Adding
>
>      def __len__(self):
>          raise NotImplemented
>
> to our subclass should do the job.  It looks like list() respects that,
> calls __iter__(), and does the right thing.  I can't find any place
> where that behavior for list() is documented,

It is a cpython implementation detail that probably has changed with the 
versions.

> but it's logical and experimentally, it seems to work.

> Can anybody see any downside to this?

No. Give it a try.


-- 
Terry Jan Reedy

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Thread

Interesting list() un-optimization Roy Smith <roy@panix.com> - 2013-03-06 22:20 -0500
  Re: Interesting list() un-optimization Dave Angel <davea@davea.name> - 2013-03-06 22:38 -0500
  Re: Interesting list() un-optimization Tim Chase <python.list@tim.thechases.com> - 2013-03-06 21:57 -0600
  Re: Interesting list() un-optimization Kev Dwyer <kevin.p.dwyer@gmail.com> - 2013-03-07 07:31 +0000
  Re: Interesting list() un-optimization Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de> - 2013-03-07 11:22 +0000
  Re: Interesting list() un-optimization Ian Kelly <ian.g.kelly@gmail.com> - 2013-03-07 09:00 -0700
  Re: Interesting list() un-optimization Christian Heimes <christian@python.org> - 2013-03-07 17:20 +0100
  Re: Interesting list() un-optimization Ian Kelly <ian.g.kelly@gmail.com> - 2013-03-07 10:31 -0700
  Re: Interesting list() un-optimization Stefan Behnel <stefan_ml@behnel.de> - 2013-03-07 20:19 +0100
  Re: Interesting list() un-optimization Ian Kelly <ian.g.kelly@gmail.com> - 2013-03-07 13:26 -0700
  Re: Interesting list() un-optimization Terry Reedy <tjreedy@udel.edu> - 2013-03-07 15:29 -0500
  Re: Interesting list() un-optimization Terry Reedy <tjreedy@udel.edu> - 2013-03-07 15:34 -0500
  Re: Interesting list() un-optimization Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de> - 2013-03-07 20:41 +0000
  Re: Interesting list() un-optimization Terry Reedy <tjreedy@udel.edu> - 2013-03-07 17:53 -0500
  Re: Interesting list() un-optimization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-03-08 17:59 +0000
    Re: Interesting list() un-optimization Roy Smith <roy@panix.com> - 2013-03-08 13:45 -0500
  Re: Interesting list() un-optimization Roy Smith <roy@panix.com> - 2013-03-10 09:05 -0400
    Re: Interesting list() un-optimization Terry Reedy <tjreedy@udel.edu> - 2013-03-10 17:39 -0400
      Re: Interesting list() un-optimization Roy Smith <roy@panix.com> - 2013-03-10 18:34 -0400
        Re: Interesting list() un-optimization Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-03-10 23:35 +0000
          Re: Interesting list() un-optimization Roy Smith <roy@panix.com> - 2013-03-10 19:50 -0400

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