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From: Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de>
Subject: Re: Interesting list() un-optimization
Date: Thu, 7 Mar 2013 20:41:37 +0000 (UTC)
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> >>> 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/
> >

very interesting (hadn't heard of it)! Just checked the PEP,
then tested list()'s behavior, and it is just as described:

class stupid(list):
	def __len__(self):
		print ('len() called')
		return NotImplemented
	
	def __length_hint__(self):
		print ('hint requested')
		l=iter(self).__length_hint__()
		print (l)
		return l

a=stupid((1,2,3))
len(d)
======>
  len() called

  Traceback (most recent call last):
    File "<pyshell#79>", line 1, in <module>
      len(d)
  TypeError: an integer is required

list(d)
======>
  len() called
  hint requested
  3
  [1, 2, 3]

so list() first tries to call the iterable's __len__ method. If that raises a
TypeError it falls back to __length_hint__ .
What I still don't know is how the listiterator object's __length_hint__ works.
Why, in this case, does it know that it has a length of 3 ? The PEP does not
provide any hint how a reasonable hint could be calculated.

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

a very good question.

Best,
Wolfgang