On Aug 25, 2017, at 12:38 PM, xoviat <xoviat@gmail.com> wrote:

According to the documentation, NotImplemented isn't appropriate either, as is for binary operations only. There is no one value that's taylor made for this situation, but an exception may be more appropriate as the underlying cause is probably an error.

The underlying cause is *not* an error just the same as it’s not an error for a __eq__ to not know how to test equality against a specific `other`. The underlying cause is explicitly “I do not want to or know how to handle this case” not “I’ve accidentally called some code that wasn’t implemented yet”. The use case is almost exactly the same as the binop use case.