Always call super in init

[NeilGirdhar]

HasTraits.__init__ does not call super. It must call super if you want to use HasTraits in multiple inheritance.

HasTraits also silently swallows positional arguments. Why does it do that?

Finally, it would be nice if HasTraits.__init__ did not automatically set its keyword arguments onto the object. The problem with this is it means that HasTraits must be the final class in the mro, which is hard to guarantee. Is this behaviour really necessary?

[NeilGirdhar]

@rmorshea These are bugs that need to be fixed please by the way. This is a basic requirement of multiple inheritance in Python.

[rmorshea]

1. I think super().__init__ is left out because HasTraits inherits from object.
2. If I'm right this change in pass *args to __new__ #173 is the positional argument problem you mentioned?
3. Value setting from kwargs in HasTraits.__init__ isn't something I'm attached to, but others probably are, and I imagine it'd represent a pretty big API breakage that would have to come in v5.0 if it were to come at all.

[NeilGirdhar]

1. That doesn't matter. A class can inherit from HasTraits and something else, which is why you must call super. E.g.

   class B:
      def __init__(self, *, x, **kwargs):
          super().__init__(**kwargs)
          self.y = f(x)
   
   class A(HasTraits, B):
       pass
   

   is broken. B.__init__ is never called.

2. Yes, thank you!

3. This breaks multiple inheritance. I personally don't think it's good Python. The only class that does this is dict, and I think that was a mistake that's unfortunately too late to fix. However, it is not too late for traitlets.

[rmorshea]

@NeilGirdhar - see #174

[rmorshea]

As for the value setting with kwargs, I'd create a separate issue for it and ping minrk, ellisonbg, and SylvainCorlay. I'm not sure what their reasons have been for keeping it so I can't speak for them on that.

[NeilGirdhar]

@rmorshea Okay thanks. Note that unless you forward the kwargs to the super call in #174, my example above is still broken since x will be swallowed before it ever reaches B.__init__.

[rmorshea]

@NeilGirdhar, unfortunately you can't do that since object.__init__ doesn't accept any args or kwargs.

[NeilGirdhar]

@rmorshea You just forward the args and kwargs. If none of the superclasses consume them, then you're right that object.__init__ will raise. If there are none left, then there is no problem. This is how cooperative multiple inheritance works in Python.

[minrk]

Args are passed through to super in #175