[Python-Dev] PEP 0424: A method for exposing a length hint (original) (raw)

M Stefan mstefanro at gmail.com
Mon Jul 16 21:36:21 CEST 2012

On 7/16/2012 9:54 AM, Stefan Behnel wrote:

Mark Shannon, 15.07.2012 16:14:

Alex Gaynor wrote:

CPython currently defines an _lengthhint_ method on several types, such as various iterators. This method is then used by various other functions (such as map) to presize lists based on the estimated returned by Don't use "map" as an example. map returns an iterator so it doesn't need lengthhint Right. It's a good example for something else, though. As I mentioned before, iterators should be able to propagate the length hint of an underlying iterator, e.g. in generator expressions or map(). I consider that an important feature that the protocol must support. Stefan

Python-Dev mailing list Python-Dev at python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/mstefanro%40gmail.com map() is quite problematic in this matter, and may actually benefit from the existence of length_hint. It is very easy to create an infinite loop currently by doing stuff like x=[1]; x+=map(str,x) [61081 refs]

x=[1]; x+=map(str,x) Traceback (most recent call last): ... MemoryError [120959834 refs] len(x) 120898752

Obviously, this won't cause an infinite loop in Python2 where map is non-lazy. Also, this won't work for all mutable containers, because not all of them permit adding elements while iterating:

s=set([1]); s.update(map(str,s)) Traceback (most recent call last): ... RuntimeError: Set changed size during iteration [61101 refs] s {1, '1'} [61101 refs] del s [61099 refs]

If map objects were to disallow changing the size of the container while iterating (I can't really think of an use-case in which such a limitation would be harmful), it might as well be with length_hint.

Also, what would iter([1,2,3]).length_hint() return? 3 or unknown? If 3, then the semantics of l=[1,2,3]; l += iter(l) will change (infinite loop without length_hint vs. list of 6 elements with length_hint). If unknown, then it doesn't seem like there are very many places where length_hint can return anything but unknown.

Regards, Stefan M

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