A special value which should be returned by the binary special methods
(e.g. __eq__(), __lt__(), __add__(), __rsub__(),
etc.) to indicate that the operation is not implemented with respect to
the other type; may be returned by the in-place binary special methods
(e.g. __imul__(), __iand__(), etc.) for the same purpose.
It should not be evaluated in a boolean context.
NotImplemented is the sole instance of the types.NotImplementedType type.
Note
When a binary (or in-place) method returns NotImplemented the
interpreter will try the reflected operation on the other type (or some
other fallback, depending on the operator). If all attempts return
NotImplemented, the interpreter will raise an appropriate exception.
Incorrectly returning NotImplemented will result in a misleading
error message or the NotImplemented value being returned to Python code.
See Implementing the arithmetic operations for examples.
Note
NotImplementedError and NotImplemented are not interchangeable,
even though they have similar names and purposes.
See NotImplementedError for details on when to use it.
Changed in version 3.9: Evaluating NotImplemented in a boolean context is deprecated. While
it currently evaluates as true, it will emit a DeprecationWarning.
It will raise a TypeError in a future version of Python.
