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:
- Why was it decided that the
True? It feels unpythonic.
- Is there a good reason I am unaware of? The documentation seems to just say, "because it is".
- 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.
__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.
__iand__(), etc.) for the same purpose. Its truth value is true.
— From the Python Docs
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.