epiphany

[Steven D'Aprano]

On Thu, 25 Apr 2013 08:36:34 -0400, Roy Smith wrote:

> In article <5178b1db$0$29977$c3e8da3$5496439d at news.astraweb.com>,
>  Steven D'Aprano <steve+comp.lang.python at pearwood.info> wrote:
> 
>> The semantics of NotImplemented is that it is a signal for one object
>> to say "I don't know how to do this, let somebody else try".
> 
> That's precisely the logic here.  The rule says, "I don't know how to
> tell you if this is OK or not, ask another rule".

Sounds good to me then. It looks like your design is actually much closer 
to what I believe the standard Python semantics are intended to be than 
it appeared at first.



[...]
> Here's what the docs say about NotImplemented:
> 
>> This type has a single value. There is a single object with this value.
>> This object is accessed through the built-in name NotImplemented.
>> Numeric methods and rich comparison methods may return this value if
>> they do not implement the operation for the operands provided. (The
>> interpreter will then try the reflected operation, or some other
>> fallback, depending on the operator.) Its truth value is true.
> 
> It gives an example of a use by numeric methods.  It doesn't say that's
> the only thing it can be used for.

Right. You can do a lot of things in Python, including shooting your foot 
off :-) but that doesn't mean you should. The further away from standard 
Python conventions you get, the more wary you should be. That's all.


> It also says, "Its truth value is true".  Why would they document that
> fact if you weren't supposed to use it as a boolean operand?

You can use *anything* in Python in a boolean context. That's a language 
feature: all objects are either truthy or falsey. As for why it is 
documented for NotImplemented, I guess that's because some people might 
guess that it is falsey, like None.



-- 
Steven