Plugin system

[Dan Perl]

"Peter Hansen" <peter at engcorp.com> wrote in message 
news:Rpudnd4nENKWMB7cRVn-rQ at powergate.ca...
> Reinhold Birkenfeld wrote:
>> Irmen de Jong wrote:
>>>Reinhold Birkenfeld wrote:
>>>
>>>>How would you implement a plugin system (for a web-based application)?
>>>
>>>This is so general a question that I cannot think of anything concrete to 
>>>answer.
>>>Please ask again, with more detail about what you want/need/have in mind.
>>
>> Okay, the question is too open: My focus is on how you would structure
>> the plugin code (directories, files) and how you would write code to
>> load these plugins.
>
> And _this_ sounds like a combination of something that's more
> a design issue (the structure) and a trivial implementation
> detail (trivial in Python, anyway).
>
> I'll try anyway, as a first test case:  put all plugins in a
> standard folder, with nothing else present in that folder.
> They can be individual modules or packages as you will.
> The code to load the plugin consists of an __import__...
> and not much else.

I have done something similar, with a few differences.  I am using a 
convention that all the plugins are implemented as a class with a specific 
name ("Handler" in my case), each plugin in its own module (so each Handler 
class is scoped by its module).  I import all the modules in the directory 
dedicated to plugins and I do the following checks: the module contains a 
class Handler, the module is not the one implementing the base class itself 
(bit of a hack, I admit), and the Handler class is a subclass of the base 
class (use isinstance).  The checks allow to have also other modules in the 
plugins directory that do not actually implement a plugin (for instance, I 
have a mixin class in that same directory).

Because you have both plugins and sub-plugins you may want to use some 
checks like mine, so you have both plugins and sub-plugins in the same 
directory, but you select only the plugin classes.  That's if I understand 
your requirements correctly.

Dan

> Maybe this feedback will help you refine the question further...
>
> -Peter