Interesting list() un-optimization
Wolfgang Maier
wolfgang.maier at biologie.uni-freiburg.de
Thu Mar 7 06:22:56 EST 2013
Tim Chase <python.list <at> tim.thechases.com> writes:
> On 2013-03-06 22:20, Roy Smith wrote:
> > I stumbled upon an interesting bit of trivia concerning lists and
> > list comprehensions today.
>
> A little testing
> shows that this can be rewritten as
>
> my_objects = list(iter(my_query_set))
>
> which seems to then skip the costly __len__ call. Performance geeks
> are welcome to time it against the list-comprehension version
>
> class Foo(object):
> def __init__(self):
> self.items = range(10)
> def __iter__(self):
> return iter(self.items)
> def __len__(self):
> print "Calling costly __len__"
> return len(self.items)
>
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. In
most cases, such a workaround will not be feasible. Why should iter(QuerySet())
have a faster __len__() method defined than QuerySet() itself. Most likely,
iter(QuerySet()) just returns self anyway?
Best,
Wolfgang
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