Add a callable traitlet type

[yuvipanda]

This would be useful when you need to pass in a function. I've run into this several times when writing JupyterHub spawners. Environment variables can be specified as a dict whose value can be callable in JupyterHub, for example. I'm implementing this in one of my spawners just now, and think it'll be useful in the core library too.

Would people be open to a patch that added a callable traitlet type? if so, should it support further validation? The only one I could think of that'll make sense in Python would be an arity check...

[yuvipanda]

I actually instead solved it like jupyterhub/systemdspawner@8d9fbc8 instead, which exposed a much smaller customization surface. This helps a lot when maintaining backwards compatibility, so perhaps not having a callable is a good idea! :)

[minrk]

I'd be fine adding callable. It's also achievable with Any + validation:

class C(HasTraits):
    f = Any()
    @validate('f')
    def _(self, proposal):
        if not callable(proposal.value):
            raise TraitError("I need a callable!")
        return proposal.value

[twavv]

Seems that the Callable trait isn't available in traitlets via pypi nor is it detailed in the documentation. Is that an oversight and/or is there any way to rectify that?

[danielballan]

I added it to the docs here #523