[Python-ideas] Adding collections.abc.Ordered

[Ben Finney]

Guido van Rossum <guido at python.org> writes:

> No, raising NotImplementedError means that a subclass was supposed to
> implement the method. In this case it's different -- it should appear
> as if the method isn't implemented to code that checks for the
> method's presence.

Understood, thanks for explaining the difference.

> Introspecting whether the code raises NotImplementedError is
> unfeasible. We explicitly decided that setting the method to None
> indicates that it should be considered as absent by code that checks
> for the method's presence.

Oh, you mean setting the attribute so it's not a method at all but a
simple non-callable object? That makes sense.

Why recommend ‘None’, though? We now have the ‘NotImplemented’ object;
why not set the attribute of the class as ‘foo = NotImplemented’?

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Ben Finney