[Python-checkins] bpo-37913: document that __length_hint__ can return NotImplemented (GH-15383)

[Steve Dower]

https://github.com/python/cpython/commit/009ef2955d91e04f62a7159a1c7ddaddbfedc3b6
commit: 009ef2955d91e04f62a7159a1c7ddaddbfedc3b6
branch: master
author: Jeroen Demeyer <J.Demeyer at UGent.be>
committer: Steve Dower <steve.dower at python.org>
date: 2019-09-10T15:01:13+01:00
summary:

bpo-37913: document that __length_hint__ can return NotImplemented (GH-15383)

files:
M Doc/reference/datamodel.rst

diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 8813f57587f0..6b9325da2f39 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -2155,8 +2155,10 @@ through the container; for mappings, :meth:`__iter__` should be the same as
 
    Called to implement :func:`operator.length_hint`. Should return an estimated
    length for the object (which may be greater or less than the actual length).
-   The length must be an integer ``>=`` 0. This method is purely an
-   optimization and is never required for correctness.
+   The length must be an integer ``>=`` 0. The return value may also be
+   *NotImplemented*, which is treated the same as if the ``__length_hint__``
+   method didn't exist at all. This method is purely an optimization and is
+   never required for correctness.
 
    .. versionadded:: 3.4