Question

I’m not looking so much for language-specific answers, just general models for implementing a plugin system (if you want to know, I’m using Python). I have my own idea (register callbacks, and that’s about it), but I know others exist. What’s normally used, and what else is reasonable?

What do you mean by a plugin system? Does Dependency Injection and IOC containers sounds like a good solution?

I mean, uh, well, a way to insert functionality into the base program without altering it. I didn’t intend to define it when I set out. Dependency Injection doesn’t look particularly suitable for what I’m doing, but I don’t know much about them.

Answer 1

There is a very good episode of Software Engineering Radio, which you may be interested in.

For future reference, I have reproduced here the ‘Rules for Enablers‘ (alternative link) given in the excellent Contributing to Eclipse by Erich Gamma, Kent Beck.

• Invitation Rule – Whenever possible, let others contribute to your contributions.
• Lazy Loading Rule – Contributions are only loaded when they are needed.
• Safe Platform Rule – As the provider of an extension point, you must protect yourself against misbehavior on the part of extenders.
• Fair Play Rule – All clients play by the same rules, even me.
• Explicit Extension Rule – Declare explicitly where a platform can be extended.
• Diversity Rule – Extension points accept multiple extensions.
• Good Fences Rule – When passing control outside your code, protect yourself.
• Explicit API Rule – separate the API from internals.
• Stability Rule – Once you invite someone to contribute, don?t change the rules.
• Defensive API Rule – Reveal only the API in which you are confident, but be prepared to reveal more API as clients ask for it.