Give a reference to a python instance attribute at class definition

Question

I have a class with attributes which have a reference to another attribute of this class. See class Device, value1 and value2 holding a reference to interface:

class Interface(object):
    def __init__(self):
        self.port=None

class Value(object):
    def __init__(self, interface, name):
        self.interface=interface
        self.name=name

    def get(self):
        return "Getting Value \"%s\" with interface \"%s\""%(self.name, self.interface.port)

class Device(object):
    interface=Interface()
    value1=Value(interface, name="value1")
    value2=Value(interface, name="value2")

    def __init__(self, port):
        self.interface.port=port

if __name__=="__main__":
    d1=Device("Foo")
    print d1.value1.get() # >>> Getting Value "value1" with interface "Foo"
    d2=Device("Bar")
    print d2.value1.get() # >>> Getting Value "value1" with interface "Bar"
    print d1.value1.get() # >>> Getting Value "value1" with interface "Bar"

The last print is wrong, cause d1 should have the interface "Foo". I know whats going wrong: The line interface=Interface() line is executed, when the class definition is parsed (once). So every Device class has the same instance of interface.

I could change the Device class to:

class Device(object):
    interface=Interface()
    value1=Value(interface, name="value1")
    value2=Value(interface, name="value2")

    def __init__(self, port):
        self.interface=Interface()
        self.interface.port=port

So this is also not working: The values still have the reference to the original interface instance and the self.interface is just another instance...

The output now is:

>>> Getting Value "value1" with interface "None"
>>> Getting Value "value1" with interface "None"
>>> Getting Value "value1" with interface "None"

So how could I solve this the pythonic way? I could setup a function in the Device class to look for attributes with type Value and reassign them the new interface. Isn't this a common problem with a typical solution for it?

Thanks!

Answer 1

interface is a class attribute. So when you do

d2=Device("Bar")

you are changing the port of the interface for all objects of class Device.

---

If you want to have these attributes per object instance, you have to put them into the __init__ method:

class Device(object):
    def __init__(self, port):
        self.interface=Interface()
        self.value1=Value(self.interface, name="value1")
        self.value2=Value(self.interface, name="value2")
        self.interface.port=port

---

You should only use class attributes if you really want to share a value with all the objects.

Other than that, I don't know what you try to accomplish here. Why defining it as class attribute in the first place?

Answer 2

You can't defer the interface lookup unless your values are methods of Device. Perhaps something like this does what you want?

def value(name):
    def get(self):
        print 'Getting %s from port %s' % (name, self.interface.port)
    def set(self, v):
        print 'Setting %s to %s on port %s' % (name, v, self.interface.port)
    return property(get, set)

class Device(object):
    value1 = value('value1')
    value2 = value('value2')
    def __init__(self, interface):
        self.interface = interface

class Interface(object):
    def __init__(self, port):
        self.port = port

d = Device(Interface(1234))
d.value1
d.value2 = 42

If you want your values to do more than get and set, then it's more difficult.