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Re: Interesting list() un-optimization

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From Stefan Behnel <stefan_ml@behnel.de>
Subject Re: Interesting list() un-optimization
Date Thu, 07 Mar 2013 20:19:07 +0100
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Ian Kelly, 07.03.2013 18:31:
> On Thu, Mar 7, 2013 at 9:20 AM, Christian Heimes wrote:
>> Am 07.03.2013 17:00, schrieb Ian Kelly:
>>> On Thu, Mar 7, 2013 at 4:22 AM, Wolfgang Maier wrote:
>>>> Well, it skips the costly len() call because your iter(Foo()) returns
>>>> iter(range()) under the hood and list() uses that object's __len__() method.
>>>
>>> Iterators do not generally have __len__ methods.
>>>
>>> >>> len(iter(range(10)))
>>> Traceback (most recent call last):
>>>   File "<stdin>", line 1, in <module>
>>> TypeError: object of type 'range_iterator' has no len()
>>
>> But iterators have a length hint method that are used for some
>> optimizations and preallocations, too.
>>
>> >>> i = iter(range(10))
>> >>> i.__length_hint__()
>> 10
>>
>> See http://www.python.org/dev/peps/pep-0424/
> 
> Didn't know about that, thanks.  Presumably a proper iter(QuerySet())
> object could implement __length_hint__ in an efficient manner rather
> than by just calling the __len__ of the underlying QuerySet,

And how exactly would it do that, without either doing what __len__ does or
reading the whole result set into memory?

Stefan

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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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