[Twisted-Python] twisted plugins

[Gabriel Rossetti]

Jean-Paul Calderone wrote:
> On Mon, 03 Mar 2008 14:44:12 +0100, Gabriel Rossetti 
> <mailing_lists at evotex.ch> wrote:
>> Hello,
>>
>> I tried creating some test plugins using the plugin howto on the 
>> twisted website. I did as shown, I added the plugins dir to the 
>> PYTHONPATH and tested it. The problem is I get nothing back when 
>> calling getPlugins(IMyInterface).  Here is some code :
>>
>> ---------------------------------------------------------------------------------------------- 
>>
>>
>>    from zope.interface import Interface
>>
>>    class IHelloWorld(Interface):
>>        """
>>        A simple test plugin that displays "Hello world"
>>        """
>>              def display():
>>            """
>>            Displays on stdout "Hello world" in a language chosen by the
>>    programmer
>>            """
>>
>> ---------------------------------------------------------------------------------------------- 
>>
>>
>>    from twisted.plugin import IPlugin
>>    from zope.interface import implements
>>    from interfaces import ihelloworld
>>
>>    class EnglishHelloWorld(object):
>>              implements(IPlugin, ihelloworld.IHelloWorld)
>>
>>        def display(self):
>>            """
>>            Displays on stdout "Hello world" in English
>>            """
>>            print "Hello world!"
>>
>> ---------------------------------------------------------------------------------------------- 
>>
>>
>>    from twisted.plugin import getPlugins
>>    from twisted.plugin import IPlugin
>>    from interfaces import ihelloworld
>>
>>    def displayHelloWorld():
>>              for h in getPlugins(ihelloworld.IHelloWorld):
>>            h().display()
>>              if(__name__ == "__main__"):
>>        displayHelloWorld()
>>
>> ---------------------------------------------------------------------------------------------- 
>>
>>
>> I tried running in debug mode to see what happens, but I don't get 
>> it, in the getPlugins() code, it does this :
>>
>>    allDropins = getCache(package)
>>
>> and in this dict I see my plugins, but their inner "plugin" member is 
>> an empty list. I looked at the code of some of the examples, they 
>> used "classProvides()" and not "implements()", so I tried that :
>>
>> ---------------------------------------------------------------------------------------------- 
>>
>>
>>    from twisted.plugin import IPlugin
>>    from zope.interface import implements
>>    from zope.interface import classProvides
>>    from interfaces import ihelloworld
>>
>>    class EnglishHelloWorld(object):
>>              classProvides(IPlugin, ihelloworld.IHelloWorld)
>>
>>        def display(self):
>>            """
>>            Displays on stdout "Hello world" in English
>>            """
>>            print "Hello world!"
>>
>> ---------------------------------------------------------------------------------------------- 
>>
>>
>> which works, is the doc out of sync with the current code or did I do 
>> something wrong? Another question, it's not really very practical to 
>> have to iterate all the plugins and I guess test to see which one is 
>> which, is there a better way of doing it?
>
> Using classProvides can be correct, although it's not quite in your 
> example
> code because EnglishHelloWorld must be instantiated in order to use 
> it, and
> that isn't reflected in your IHelloWorld interface.  You could make 
> display
> a static or class method.  Or you continue using implements and 
> instantiate
> EnglishHelloWorld, binding a name to the result (as the documentation 
> shows
> with steelPlate/brassPlate).
>
> In the example, when the plugin system finds objects, it finds steelPlate
> and brassPlate, not SimpleMaterial.
>
> Jean-Paul
>
Thank you Jean-Paul, I understand better now. I had been trying to 
create instances in the main. There's one thing I don't get though with 
those 2 lines, if I put them in the main, before the for loop, I get en 
error that states that it can't be instantiated, but it works in the 
class, how come?

Gabriel