[Python-Dev] PEP 457: Syntax For Positional-Only Parameters (original) (raw)

Ethan Furman ethan at stoneleaf.us
Wed Oct 9 02:38:05 CEST 2013

On 10/08/2013 05:15 PM, Steven D'Aprano wrote:

On Wed, Oct 09, 2013 at 01:33:26AM +0200, Larry Hastings wrote:

The obvious solution: add a new singleton constant to Python that is passed in when a parameter is not mapped to an argument. I propose that the value be called called undefined, and be a singleton of a special class called Undefined. If a positional-only parameter did not receive an argument when called, its value would be set to undefined. I would much prefer Undefined and UndefinedType. That matches other singletons like None, NotImplemented, Ellipsis, even True and False.

The type of Ellipsis is ellipsis, and the type of True and False is bool. ;) That aside, I agree that Undefined and UndefinedType would be better.

What (if any) methods and attributes would Undefined have? If it doesn't have any, doesn't that make it functionally equivalent to None? Why not just use None? The PEP needs to explain why it needs to invent yet another singleton that quacks like None.

I think the key question here is are there places where None is the passed in value, and its meaning is something besides "no value passed in". I don't know the answer.

[Bikeshed: perhaps Missing is more appropriate than Undefined? After all, the parameter is defined, only the value is missing.]

Or the value is undefined. ;)

-- Ethan

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