Maxime Liquet

There's been no deprecation warning added to `set_hook` and [it is documented](https://param.holoviz.org/user_guide/Parameter_Types.html?highlight=set_hook#numbers). Yet, still a good idea to remove it!

Added a deprecation warning in https://github.com/holoviz/param/issues/692

Somewhat related to https://github.com/holoviz/param/pull/425

Thanks for the issue report, I can indeed reproduce it with this simplified version: ```python import param class Other(param.Parameterized): x = param.Integer(1) other = Other() class P(param.Parameterized): other = param.Parameter(other)...

Selectors need more work in general, that change can happen when this work is undertook, as as is it would not fix anything, except my need for consistency :). Moving...

I see this is allowing pathlib.Path paths, which is great, but I'd like Param to accept that consistently where it makes sense and not just in some Parameters. Also, if...

Considering the API only of this feature, there are two options available with a) passing a sentinel value to `default` (`Parameter(default=REQUIRED)`) and b) adding a new `required` slot (`Parameter(required=True)`), b)...

I have updated this PR, most importantly to make sure all the class Parameters are evaluated. I have also added some more tests and updated the existing ones. The behavior...

I see your point. I think the point made by Marc on [Discourse](https://discourse.holoviz.org/t/how-to-make-parameter-values-mandatory/5124) is that he wanted to avoid having to write boiler plate code. What this PR doesn't address...

@jlstevens a *potential* advantage of having `required` as a new slot is that it could be leveraged by GUIs to label a widget as "mandatory" (with that red asterisk, you...