class_traits uses __base_traits__ instead of __class_traits__

[kitchoi]

I came across the following base class and subclass:

class Base(HasTraits):
    a = Str("a")

class Child(Base):
    a = "overridden"

If you try to inspect the default value for Child.a:

> Child.class_traits()["a"].default
"a"

But I'd expect "overridden" to be returned instead.

If I access __class_traits__ instead, I get the value I expected:

> Child.__class_traits__["a"].default
"overridden"

(It was new to me that redefining the traits in a subclass is considered as redefining the traits with a new default value. See this branch.)

Turns out HasTraits.class_traits is using __base_traits__, not __class_traits__: https://github.com/enthought/traits/blob/ba7c10fa28666219e276c7eeeac7e3e7168a45c2/traits/has_traits.py#L3085-L3094

Is this intended? Or is this a bug?

[corranwebster]

My suspicion is that maybe https://github.com/enthought/traits/blob/ba7c10fa28666219e276c7eeeac7e3e7168a45c2/traits/has_traits.py#L633 should be setting into the base trait dict as well as the class trait dict? Either that or class_traits should be using __class_traits__ as you suggest. But I'm not yet sure enough which of them is "right".

[mdickinson]

One thing I'd like to do is expose the actual class_traits and instance_traits dictionaries (the ctrait_dict and itrait_dict fields on a C has_traits_object) that CHasTraits is using, if only for debugging purposes. I don't think there's currently any clean way to get those dictionaries. More generally, as much as possible of cTrait and CHasTraits should be introspectable at some level.

[mdickinson]

And as far as I can tell, __base_traits__ and __class_traits__ are intentionally different in that __base_traits__ are the ones used when subclassing. I'm not convinced that the line @corranwebster identifies should be changed, but I do think we should probably link the class_traits method to the __class_traits__ dictionary rather than the __base_traits__ dictionary. It would be helpful to go through ETS and identify uses of the class_traits method to see if any of them would be adversely affected by such a change.

[kitchoi]

Yes I was under the same impression that __base_traits__ refers to the traits defined in the base classes, whereas __class_traits__ refers to the traits on the current(?) class.

I am wondering (going down the rabbit hole)... is HasTraits.traits affected too? This line: https://github.com/enthought/traits/blob/ba7c10fa28666219e276c7eeeac7e3e7168a45c2/traits/has_traits.py#L3018

[mdickinson]

is HasTraits.traits affected too?

Likely, yes. The traits that a HasTraits instance is actually using on attribute lookup are in the ctrait_dict and itrait_dict fields; I'd expect those dictionaries to be the ones being used when reporting either class_traits or traits.