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bpo-37913: document that __length_hint__ can return NotImplemented #15383

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Sep 10, 2019
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bpo-37913: document that __length_hint__ can return NotImplemented #15383

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6 changes: 4 additions & 2 deletions Doc/reference/datamodel.rst
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Expand Up @@ -2144,8 +2144,10 @@ 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__``
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Suggested change
*NotImplemented*, which is treated the same as if the ``__length_hint__``
: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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