support default value in union types

[eli-bl]

Currently, if you declare a property like this—

        multiTypeProp:
          type:
            - string
            - integer
            - boolean
          default: abc

—or, equivalently (they end up becoming the same thing in the parser logic)—

        multiTypeProp:
          oneOf:
            - type: string
            - type: integer
            - type: boolean
          default: abc

—the default value is simply ignored. UnionProperty.build takes it as a parameter, but discards it.

However, UnionProperty.convert_value is implemented to validate a default value against each of the variants. It just isn't getting called. So, if build actually called convert_value at the end, as we do with many other property types, it looks to me like it would work.

It would still have a few limitations due to how defaults are implemented in general:

• It would only work if the variants are scalar types; ModelProperty and ListProperty don't currently support defaults.
• In my example above, if you changed the default to an integer 123 or a boolean true, you would not actually get that value; instead it would convert them to strings. That's because string appears first in the variant list and StringProperty.convert_value intentionally coerces non-string types. (I'm not sure that that coercion is always desirable, but changing it would definitely be a breaking change... and this is one area where JSON Schema is fairly vague about what the correct behavior is.) Anyway, it would work as expected if you put the string type at the end of the list instead.

[eli-bl]

Oh, good grief, never mind - I was looking at the wrong thing, the default value is being applied. The limitations I described still apply but that would be a separate discussion.