[Python-Dev] Supporting functools.singledispatch with classes. (original) (raw)

Ethan Smith ethan at ethanhs.me
Sun Dec 24 21:32:08 EST 2017

Hello all,

In https://bugs.python.org/issue32380 I was hoping to add support for singledispatch with methods. Unfortunately, this cannot be achieved internally without ugly attribute or stack hacks.

Therefore, I was thinking it would be nice if singledispatch supported a keyword argument of the argument index to dispatch on, thus one can say:

class A: @singledispatch(arg=1) def method(self, a): return 'base' @method.register(int) def method(self, a): return 'int'

The other option that could work is to define a special decorator for methods

def methodsingledispatch(func): """Single-dispatch generic method decorator.""" wrapped = singledispatch(func) def wrapper(*args, **kw): return wrapped.dispatch(args[1].class)(*args, **kw) wrapper.register = wrapped.register update_wrapper(wrapper, func) return wrapper

Since this is an API change, Ivan said I should post here to get feedback.

I prefer the first design, as it is more generic and flexible.

There is also the issue of classmethod and staticmethod. Since these are descriptors, I'm not sure they will work with singledispatch at all.

if you do

@classmethod @singledispatch def foo(cls, arg): ...

You lose register on foo, breaking everything. I believe this would require changing classmethod thus is a non-starter.

If you do

@singledispatch @classmethod def foo(arg): ...

The wrapper in singledispatch needs to be called as the func in classmethod, but func is readonly.

So at the moment, I don't think it is possible to implement singledispatch on classmethod or staticmethod decorated functions.

I look forward to people's thoughts on these issues.

Cheers,

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