Re: [Python-Dev] Proper way to specify that a method is not defined for a type

For those interested in this topic, if you are not already aware of it, see also http://bugs.python.org/issue25958, which among other things has a relevant proposed patch for datamode.rst. On Tue, 07 Jun 2016 10:56:37 -0700, Guido van Rossum wrote:

Setting it to None in the subclass is the intended pattern. But CPython must explicitly handle that somewhere so I don't know how general it is supported. Try defining a list subclass with __len__ set to None and see what happens. Then try the same with MutableSequence.

On Tue, Jun 7, 2016 at 10:37 AM, Ethan Furman wrote:

...

For binary methods, such as __add__, either do not implement or return NotImplemented if the other operand/class is not supported.

For non-binary methods, simply do not define.

Except for subclasses when the super-class defines __hash__ and the subclass is not hashable -- then set __hash__ to None.

Question:

Are there any other methods that should be set to None to tell the run-time that the method is not supported? Or is this a general mechanism for subclasses to declare any method is unsupported?