Why is NotImplemented truthy in Python 3?

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This question is spurred from the answers and discussions of this question. The following snippet shows the crux of the question:

>>> bool(NotImplemented) True 

The questions I have are the following:

  1. Why was it decided that the bool value of NotImplemented should be True? It feels unpythonic.
  2. Is there a good reason I am unaware of? The documentation seems to just say, "because it is".
  3. Are there any examples where this is used in a reasonable manner?

Reasoning behind why I believe it's unintuitive (please disregard the lack of best practice):

>>> class A: ...     def something(self): ...         return NotImplemented ... >>> a = A() >>> a.something() NotImplemented >>> if a.something(): ...     print("this is unintuitive") ... this is unintuitive 

It seems an odd behavior that something with such a negative connotation (lack of implementation) would be considered truthy.

Relevant text from:


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. Its truth value is true.

— From the Python Docs

Edit 1

To clarify my position, I feel that NotImplemented being able to evaluate to a boolean is an anti-pattern by itself. I feel like an Exception makes more sense, but the prevailing idea is that the constant singleton was chosen for performance reasons when evaluating comparisons between different objects. I suppose I'm looking for convincing reasons as to why this is "the way" that was chosen.


By default, an object is considered truthy (bool(obj) == True) unless its class provides a way to override its truthiness. In the case of NotImplemented, no one has ever provided a compelling use-case for bool(NotImplemented) to return False, and so <class 'NotImplementedType'> has never provided an override.


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