`allow_None` not enforced when set to `False` and the default value is `None`

[maximlt]

Setting allow_None to False when the default Parameter value is None automatically sets it to True. I believe Param shouldn't be doing that, when allow_None is explicitly set to False this value should be enforced. The annoying bit being that for Parameters whose default default value is None (like Callable below) you'd have to provide a value that is not None. Unless this applies only after the Parameter has been instantiated, to allow it to be instantiated with None as the value for default with allow_None=False, but preventing it to be set to None afterwards, which I believe is what you can do now with ObjectSelector.

import param

class A(param.Parameterized):
    c = param.Callable(allow_None=False)

print(A.param.c.allow_None)
# True!

[MarcSkovMadsen]

Just got hit by this too. Would expect the below to raise an exception when trying to instantiate the Viewer. Not before.

import param

class Viewer(param.Parameterized):
    data = param.DataFrame(allow_None=False)

viewer = Viewer()

As an alternative we need a required argument that makes it mandatory to specify the value on instantiation.