bpo-15718: Document the upper bound constrain on the __len__ return value. (#1256)
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 095a238..35925a0 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -2011,6 +2011,14 @@
:meth:`__bool__` method and whose :meth:`__len__` method returns zero is
considered to be false in a Boolean context.
+ .. impl-detail::
+
+ In CPython, the length is required to be at most :attr:`sys.maxsize`.
+ If the length is larger than :attr:`!sys.maxsize` some features (such as
+ :func:`len`) may raise :exc:`OverflowError`. To prevent raising
+ :exc:`!OverflowError` by truth value testing, an object must define a
+ :meth:`__bool__` method.
+
.. method:: object.__length_hint__(self)
@@ -2021,6 +2029,7 @@
.. versionadded:: 3.4
+
.. note::
Slicing is done exclusively with the following three methods. A call like ::