[docs] [issue15997] NotImplemented needs to be documented

[Martin v. Löwis]

Martin v. Löwis added the comment:

Terry: can you propose an alternative wording? I don't think that the discussion of TypeError belongs into the description of NotImplemented, though.

IIUC, NotImplemented was added for performance reasons; returning a value is much faster than raising an exception. The return only requires an INCREF/DECREF pair, and a pointer identity test. The exception requires to create a new object, possibly a traceback, and a subclass check on exception matching.

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<http://bugs.python.org/issue15997>
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