Python – Converting an object into a subclass in Python


Lets say I have a library function that I cannot change that produces an object of class A, and I have created a class B that inherits from A.

What is the most straightforward way of using the library function to produce an object of class B?

edit- I was asked in a comment for more detail, so here goes:

PyTables is a package that handles hierarchical datasets in python. The bit I use most is its ability to manage data that is partially on disk. It provides an 'Array' type which only comes with extended slicing, but I need to select arbitrary rows. Numpy offers this capability – you can select by providing a boolean array of the same length as the array you are selecting from. Therefore, I wanted to subclass Array to add this new functionality.

In a more abstract sense this is a problem I have considered before. The usual solution is as has already been suggested- Have a constructor for B that takes an A and additional arguments, and then pulls out the relevant bits of A to insert into B. As it seemed like a fairly basic problem, I asked to question to see if there were any standard solutions I wasn't aware of.

Best Answer

This can be done if the initializer of the subclass can handle it, or you write an explicit upgrader. Here is an example:

class A(object):
    def __init__(self):
        self.x = 1

class B(A):
    def __init__(self):
        super(B, self).__init__()
    def _init_B(self):
        self.x += 1

a = A()
b = a
b.__class__ = B

assert b.x == 2