Single choice type

[yychen]

Description

Currently the types are simple. We often use string to refer to some state, but actually it should be a single choice. For example, there might be a key called system.state and there are only four possible values intro, opened, post-event and closed. If we use the str type, then it is error prone due to typo, extra spaces, first letter capitalized etc.

Possible Solutions

With the introduction of schema (#12), we might be able to define the possible values in the schema. And we can use a drop-down or single select widget in django-admin to render only possible values.

I think we can also put help text in the schema explaining what each value represents.