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

[Steve Dower]

https://github.com/python/cpython/commit/ed99bb9ca68b37cfaec3629afa67d70289f3ffc7
commit: ed99bb9ca68b37cfaec3629afa67d70289f3ffc7
branch: 3.8
author: Steve Dower <steve.dower at python.org>
committer: GitHub <noreply at github.com>
date: 2019-09-10T15:31:26+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..c6c6e4075039 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -2155,7 +2155,9 @@ 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
+   The length must be an integer ``>=`` 0. The return value may also be
+   :const:`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