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

Antoine Pitrou solipsis at pitrou.net
Mon Nov 3 18:10:59 CET 2014

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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