[Python-Dev] The role of NotImplemented: What is it for and when should it be used? (original) (raw)

Guido van Rossum guido at python.org
Mon Nov 3 19:00:24 CET 2014

That must be so that an immutable type can still implement iop as an optimization.

On Mon, Nov 3, 2014 at 9:10 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:

On Mon, 3 Nov 2014 09:05:43 -0800 Guido van Rossum <guido at python.org> wrote: > Sorry, was too quick. For immutable types iop may not exist and then > the fallback machinery should work normally using NotImplemented. But if > iop exists it can choose not to allow rop, because the type would > presumably change. This is probably more predictable. I don't even know if > the byte code interpreter looks for Not implemented from iop.

Apparently it can tell it to fallback on op: >>> class C(list): ... def iadd(self, other): ... print("here") ... return NotImplemented ... >>> c = C() >>> c += [1] here >>> c [1] >>> type(c) <class 'list'>

Regards Antoine.

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