Replacing the object from one of its methods

Question

I am using python and have an object, that object has a method. I am looking for a simple way, to replace the entire object from within that function.

E.g

class a():
    def b(self):
        self = other_object

How can you do that?

Thanks

Answer 1

You use a proxy/facade object to hold a reference to the actual object, the self if you wish and that proxy (better term than Facade, but not changing my code now) is what the rest of your codebase sees. However, any attribute/method access is forwarded on to the actual object, which is swappable.

Code below should give you a rough idea. Note that you need to be careful about recursion around __the_instance, which is why I am assigning to __dict__ directly. Bit messy, since it's been a while I've written code that wraps getattr and setattr entirely.

class Facade:
    def __init__(self, instance):
        self.set_obj(instance)

    def set_obj(self, instance):
        self.__dict__["__theinstance"] = instance

    def __getattr__(self, attrname):
        if attrname == "__theinstance":
            return self.__dict__["__theinstance"]                

        return getattr(self.__dict__["__theinstance"], attrname)

    def __setattr__(self, attrname, value):
        if attrname == "__theinstance":
            self.set_obj(value)

        return setattr(self.__dict__["__theinstance"], attrname, value)


class Test:
    def __init__(self, name, cntr):
        self.name = name 
        self.cntr = cntr

    def __repr__(self):
        return "%s[%s]" % (self.__class__.__name__, self.__dict__)

obj1 = Test("first object", 1)

obj2 = Test("second", 2)
obj2.message = "greetings"


def pretend_client_code(facade):
    print(id(facade), facade.name, facade.cntr, getattr(facade, "value", None))


facade = Facade(obj1)

pretend_client_code(facade)

facade.set_obj(obj2)

pretend_client_code(facade)

facade.value = 3

pretend_client_code(facade)

facade.set_obj(obj1)

pretend_client_code(facade)

output:

4467187104 first object 1 None
4467187104 second 2 None
4467187104 second 2 3
4467187104 first object 1 None

So basically, the "client code" always sees the same facade object, but what it is actually accessing depends on what your equivalent of def b is has done.

Facade has a specific meaning in Design Patterns terminology and it may not be really applicable here, but close enough. Maybe Proxy would have been better.

Note that if you want to change the class on the same object, that is a different thing, done through assigning self.__class__ . For example, say an RPG game with an EnemyClass who gets swapped to DeadEnemyClass once killed: self.__class__ = DeadEnemyClass

Answer 2

You can't directly do that. What you can do is save it as an instance variable.

class A():
    def __init__(self, instance=None):
        self.instance = val or self

    # yes, you can make it a property as well.
    def set_val(self, obj):
        self.instance = obj

    def get_val(self):
        return self.instance

It is unlikely that replacing the 'self' variable will accomplish whatever you're trying to do, that couldn't just be accomplished by storing the result of func(self) in a different variable. 'self' is effectively a local variable only defined for the duration of the method call, used to pass in the instance of the class which is being operated upon. Replacing self will not actually replace references to the original instance of the class held by other objects, nor will it create a lasting reference to the new instance which was assigned to it.

Original source: Is it safe to replace a self object by another object of the same type in a method?