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[3.7] bpo-37913: document that __length_hint__ can return NotImplemented (GH-15383) #15863

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Merged
zooba merged 1 commit into from
Sep 10, 2019
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[3.7] bpo-37913: document that __length_hint__ can return NotImplemented (GH-15383) #15863

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4 changes: 3 additions & 1 deletion Doc/reference/datamodel.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -2142,7 +2142,9 @@ 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
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