Question

I’m trying to create a handler which triggers whenever any of object’s attributes’ value changes. I’m still quite new with python, and didn’t really come up with any working ways. This is what I did, and I know it can’t work like this, it’s just to show what I’m trying to achieve:

# ../handler.py
import copy

class Handler:
    def __init__(self, obj):
        # Let the object know it's handler
        obj.handler = self

        # Copy the object
        self.copy = copy.deepcopy(obj)

        # Let handler know it's object
        self.obj = obj

        # Handler not running atm.
        self.running = False

def start(self):
    # Running
    self.running = True

    # As long as running...
    while self.running:

        # Loop through all objects attributes
        for attr in self.obj.__dict__:

            # If attribute's value has changed
            if self.copy.__dict__[attr] != self.obj.__dict__[attr]:

                # Print out a message
                print("%s was changed to %s" %(attr, self.obj.__dict__[attr]))

                # Update the value to our copy
                self.copy.__dict__[attr] = self.obj.__dict__[attr]

def stop(self):
    # Not running
    self.running = False

Here’s the main file

# ../main.py
from handler import Handler

class Value:
    def __init__(self, v=None):
        self.value = v

def createObject(v):
    obj = Value(v)
    Handler(obj)
    return obj

myObj = createObject(5)
myObj.handler.start()

This will, however, result into this text getting printed out: handler was changed to <Handler.Handler object at 0x0000000002B1FC50> but I’m not changing the handler here? Also, the program will freeze, due to the while loop inside the Handler.start() function… How could I implement this in a working way?

Answer 1

You effectively want a proxy, and here’s a bit of a start…

class Proxy(object):
    def __init__(self, obj):
        object.__setattr__(self, '__obj', obj)
    def __getattribute__(self, name):
        return getattr(object.__getattribute__(self, '__obj'), name)


a = range(10)
p = Proxy(a)

p.append(10) # etc...

You’ll also need to look for __setattribute__ and __delattribute__ and other magic methods…

How you’d cater for when the attribute changes could be a bit trickier – as you’d have to know if method calls existed which changed the object inplace… (such as append above)

A quick Google comes up with: http://code.activestate.com/recipes/496741-object-proxying/