[Python-Dev] PEP 443 - Single-dispatch generic functions (original) (raw)

PJ Eby pje at telecommunity.com
Thu May 23 20:59:01 CEST 2013

On Thu, May 23, 2013 at 11:11 AM, Paul Moore <p.f.moore at gmail.com> wrote:

Is the debate between 1 and 2, or 1 and 3? Is it even possible to implement 3 without having 2 different names for "register"?

Yes. You could do it as either: @func.register def doit(foo: int): ...

by checking for the first argument to register() being a function, or:

@func.register() def doit(foo: int): ...

by using a default None first argument. In either case, you would then raise a TypeError if there wasn't an annotation.

As to the ability to do multiple types registration, you could support it only in type annotations, e.g.:

@func.register
def doit(foo: [int, float]):
    ...

without it being confused with being multiple dispatch.

One other thing about the register API that's currently unspecified in the PEP: what does it return, exactly?

I generally lean towards returning the undecorated function, so that if you say:

@func.register
def do_int(foo: int):
    ...

You still have the option of calling it explicitly. OTOH, some may prefer to treat it like an overload and call it 'func' every time, in which case register should return the generic function. Some guidance as to what should be the official One Obvious Way would be helpful here.

(Personally, I usually name my methods explicitly because in debugging it's a fast clue as to which piece of code I should be looking at.)

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