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Re: Interesting list() un-optimization

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On Thu, Mar 7, 2013 at 4:22 AM, Wolfgang Maier
<wolfgang.maier@biologie.uni-freiburg.de> 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()

> In
> most cases, such a workaround will not be feasible. Why should iter(QuerySet())
> have a faster __len__() method defined than QuerySet() itself.

iter(QuerySet()) should not have any __len__ method defined at all,
which is why the optimization would be skipped.

> Most likely,
> iter(QuerySet()) just returns self anyway?

But on this point, you are correct.  The mongoengine QuerySet.__iter__
method is defined as:

    def __iter__(self):
        self.rewind()
        return self

This is unfortunate design.  Not only does it mean that the iterator's
__len__ method cannot be trusted (what should the __len__ of a
partially exhausted iterator return?), but it also means that requesting
an iterator over the QuerySet will also silently invalidate any
existing iterators.

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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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