Automatically loading and initialising objects from a plugins directory

Fredrik Lundh fredrik at
Tue Jul 22 18:28:28 CEST 2008

Dave Challis wrote:

> Thanks for that, it helped as a starting point.  I had some trouble with
> using issubclass though (issubclass(Plugin, Plugin) returns true), which
> was complicating things.
> I modified your code to the following instead (which may well have it's
> own pitfalls I'm not aware of!):
> for name in dir(plugin):
>     thing = getattr(plugin, name)
>     if hasattr(thing, '__bases__') and \
>        getattr(thing, '__bases__')[0] == Plugin:
>         thing()

so now you're no longer supporting mixins and multiple-level 
inheritance?  why not just tweak Diez' example a little:

for name in dir(plugin):

     thing = getattr(plugin, name)

     # make sure this is a plugin
        if thing is Plugin:
        if not issubclass(thing, Plugin):
     except ValueError: # issubclass sucks
        continue # not a class

     thing() # probably needs error handling around this

(I also moved the thing call out of the thing validation part, to allow 
you to distinguish between ValueErrors caused by issubclass and errors 
caused by things)

(btw, another approach would be to use a metaclass to make Plugin 
classes register themselves on import)


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