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

R. David Murray rdmurray at bitdance.com
Tue Jun 7 14:45:03 EDT 2016

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 <guido at python.org> 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 <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?

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