Question

I have the following decorator, which saves a configuration file after a method decorated with @saveconfig is called:

class saveconfig(object):
    def __init__(self, f):
        self.f = f

    def __call__(self, *args):
        self.f(object, *args)
        # Here i want to access "cfg" defined in pbtools
        print "Saving configuration"

I’m using this decorator inside the following class. After the method createkvm is called, the configuration object self.cfg should be saved inside the decorator:

class pbtools()
    def __init__(self):
        self.configfile = open("pbt.properties", 'r+')
        # This variable should be available inside my decorator
        self.cfg = ConfigObj(infile = self.configfile)

    @saveconfig
    def createkvm(self):
        print "creating kvm"

My problem is that i need to access the object variable self.cfg inside the decorator saveconfig. A first naive approach was to add a parameter to the decorator which holds the object, like @saveconfig(self), but this doesn’t work.

How can I access object variables of the method host inside the decorator? Do i have to define the decorator inside the same class to get access?

Answer 1

You have to make your decorator class behave as a descriptor to be able to access the instance:

class saveconfig(object):
    def __init__(self, f):
        self.f = f

    def __get__(self, instance, owner):
        def wrapper(*args):
            print "Saving configuration"
            print instance.cfg
            return self.f(instance, *args)
        return wrapper

Your code passes object as first parameter to self.f(), where it should pass the pbtools instance.