added a key_trait argument to Dict

[burnpanck]

Fixes #304

[burnpanck]

I do think that constructor keywords are confusing as they are. I would propose to change trait->value_trait and traits to something more expressive, e.g. per_key_traits, keyed_[value_]traits or even schema.

[minrk]

To avoid backward-incompatibility with positional args, can you add key_trait to the end?

[burnpanck]

To be honest, I think this schema feature accessible via the traits argument doesn't belong into Dict at all. I'd rather not have it block access to key_trait. What about a compromise: Keep key_trait before traits and when an instance of dict (the python class, not the trait) is encountered in key_trait, raise a warning and map it to traits?

[minrk]

My preference would be for all of these after the first to be keyword-only arguments, but Python 2 doesn't make that very easy other than **kwargs. I would add it to the end and treat it as if it were keyword-only.

[burnpanck]

To me the keys of a dict are at least as important as the values if not more. I have the feeling everyone here seems to implicitly assume unicode keys for their dicts, but that is not validated in the current implementation. I'd put the key trait even before the value trait if not for backwards compatibility.

[minrk]

I'm concerned more with clarity than priority. I don't view the order as being significant (to me, there is only one positional arg for any TraitType, but Python 2 doesn't make this easy). If you'd like to make value_trait and key_trait, that would be fine by me, as long as it's done in a backward-compatible way (i.e. positional args retain their order). I wouldn't expect anyone to pass key_trait positionally ever, and I'm AOK encouraging people to pass value_trait by keyword-arg, as well, since that would be clearest.

[burnpanck]

I don't see how there is a general one positional arg that would have a well defined meaning for all TraitType's, so the positional args could equally well be trait specific. But then, I guess backwards compatibility shouldn't be broken lightly, so it's anyway too late. I'll shut up now and will just fix backwards compatibility.

[minrk]

I do think that constructor keywords are confusing as they are

If you want to add a value_trait arg and deprecate trait, that would be fine, too.

[minrk]

Thanks!