Python decorators that are part of a base class cannot be used to decorate member functions in inherited classes

Question

Python decorators are fun to use, but I appear to have hit a wall due to the way arguments are passed to decorators. Here I have a decorator defined as part of a base class (the decorator will access class members hence it will require the self parameter).

class SubSystem(object):     def UpdateGUI(self, fun): #function decorator         def wrapper(*args):             self.updateGUIField(*args)             return fun(*args)         return wrapper      def updateGUIField(self, name, value):         if name in self.gui:             if type(self.gui[name]) == System.Windows.Controls.CheckBox:                 self.gui[name].IsChecked = value #update checkbox on ui              elif type(self.gui[name]) == System.Windows.Controls.Slider:                 self.gui[name].Value = value # update slider on ui           ... 

I've omitted the rest of the implementation. Now this class is a base class for various SubSystems that will inherit from it - some of the inherited classes will need to use the UpdateGUI decorator.

class DO(SubSystem):     def getport(self, port):         """Returns the value of Digital Output port "port"."""         pass      @SubSystem.UpdateGUI     def setport(self, port, value):         """Sets the value of Digital Output port "port"."""         pass 

Once again I have omitted the function implementations as they are not relevant.

In short the problem is that while I can access the decorator defined in the base class from the inherited class by specifiying it as SubSystem.UpdateGUI, I ultimately get this TypeError when trying to use it:

unbound method UpdateGUI() must be called with SubSystem instance as first argument (got function instance instead)

This is because I have no immediately identifiable way of passing the self parameter to the decorator!

Is there a way to do this? Or have I reached the limits of the current decorator implementation in Python?

Answer 1

You need to make UpdateGUI a @classmethod, and make your wrapper aware of self. A working example:

class X(object):     @classmethod     def foo(cls, fun):         def wrapper(self, *args, **kwargs):             self.write(*args, **kwargs)             return fun(self, *args, **kwargs)         return wrapper      def write(self, *args, **kwargs):         print(args, kwargs)  class Y(X):     @X.foo     def bar(self, x):         print("x:", x)  Y().bar(3) # prints: #   (3,) {} #   x: 3 

Answer 2

It might be easier to just pull the decorator out of the SubSytem class: (Note that I'm assuming that the self that calls setport is the same self that you wish to use to call updateGUIField.)

def UpdateGUI(fun): #function decorator     def wrapper(self,*args):         self.updateGUIField(*args)         return fun(self,*args)     return wrapper  class SubSystem(object):     def updateGUIField(self, name, value):         # if name in self.gui:         #     if type(self.gui[name]) == System.Windows.Controls.CheckBox:         #         self.gui[name].IsChecked = value #update checkbox on ui          #     elif type(self.gui[name]) == System.Windows.Controls.Slider:         #         self.gui[name].Value = value # update slider on ui          print(name,value)  class DO(SubSystem):     @UpdateGUI     def setport(self, port, value):         """Sets the value of Digital Output port "port"."""         pass  do=DO() do.setport('p','v') # ('p', 'v')