Always call super in init about traitlets HOT 9 CLOSED

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

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1. I think super().__init__ is left out because HasTraits inherits from object.
2. If I'm right this change in #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.

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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.

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@NeilGirdhar - see #174

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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.

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@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__.

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

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@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.

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Args are passed through to super in #175

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