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

Guido van Rossum guido at python.org
Tue Jun 7 13:56:37 EDT 2016

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 <ethan at stoneleaf.us> 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?
> --
> ~Ethan~
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--Guido van Rossum (python.org/~guido)
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