CODEWITHSUNDEEP
pythonclassmultiple-inheritance
nls
nls

Reputation: 1

python: changing object class in multi-level inheritance

I have looked at many questions posted here to find an answer to my problem, but I wasn't successful. The problem might be, that I just don't know for what keywords I should look. So my problem is the following:

I've got a program, that has a multi-level inheritance and I am trying to figure out how the best way would be to change the class of an object to a subclass. Let's say I have the following code:

class A(object):

    def __init(self, filename, ..)
        super(A, self).__init__()
        ...some assignments here

class B(A):

    def __init(self, filename, ..)
        super(B, self).__init__()
        ...some assignments here

class C(A):

    def __init(self, filename, ..)
        super(C, self).__init__()
        ...some assignments here

and so on...
I always want to start initialising an object of class A. Depending on the type of the file that is used, the assignments may differ and depending on those assignments I can determine what kind of file it is. So now I want to change the class of the object to whatever class is suitable..

I know I could pass the A object to B or C and use copy or deepcopy, but in A i am assigning an object of which the reference should not change and some others where it should change. Also I would need to delete that object of A, after initialising B or C.

class B(A):

    def __init__(self, filename, objA = None):
        if objA is not None:
            self.__dict__ = copy.deepcopy(objA.__dict__)
            del(objA)
        else:
            super(B, self).__init__(filename)

Also there is another possibility by changing the _class attribute to another class and use some kind of update method of the new class.
I would like to know, which of the two approaches is recommended or is there even a better one. Thanks in advance.

Upvotes: 0

Views: 1835

Answers (2)

kindall
kindall

Reputation: 184161

What you want is a factory: a function that opens the file, reads the stuff it needs to read to figure out what kind of file it is, and then initializes and returns an object of the appropriate class.

If you want to keep it a class, you'd want to override __new__() and then return an object of the desired class instead of its own class. (You could also do it using a metaclass and overriding __call__() on that.)

You can change an instance's class after instantiating it as well, by changing its __class__ attribute to point to the desired class. That'll work, but the factory is going to be more familiar to other programmers who will read your code.

Upvotes: 3

Stephan
Stephan

Reputation: 3799

This code will explain what you want to do

class B(object):
    def x(self):
        print 'b'

class A(object):
    def x(self):
        print 'a'

Now we create two objects

a = a()
b = b()

a.x()
a

b.x()
b

now if you want 'a' to become a B object 

 a.__class__ = type(b)

or 

 a.__class__ = B

now the x attribute is from the B class.

 a.x()
 b

Upvotes: 1

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