new-style classes multiplication error message isn't veryinformative

[Jon Guyer]

Steven D'Aprano <steve <at> REMOVETHIScyber.com.au> writes:

> 
> On Fri, 30 Dec 2005 03:47:30 +0000, Jon Guyer wrote:
> 
> > We have a rather complicated class that, under certain circumstances, knows
> > that it cannot perform various arithmetic operations, and so returns
> > NotImplemented.  As a trivial example:
> 
> Why don't you raise the exception yourself?
> 
> (Note the difference between NotImplemented and NotImplementedError.)

Because although A may not know how to multiply B, B might know how to multiply A

> I'd never noticed the behaviour of Python where it takes a return value of
> NotImplemented and raises a ValueError. Unless somebody can tell me why
> this is justified, I consider this at best a wart. If I return something,
> that's my return value! I don't see why arithmetic operations are special
> enough to justify this special behaviour.

Python Reference Manual, Section 3.2:

----

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.

----

This is exactly the behavior we want.  Our code paths are simpler and less
error prone if A and B don't both know how to multiply with each other, and
this seems to be exactly what NotImplemented and __mul__/__rmul__ are
designed for.