Converting an object into a subclass in Python?

Question

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.

Answer 1

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__()         self._init_B()     def _init_B(self):         self.x += 1  a = A() b = a b.__class__ = B b._init_B()  assert b.x == 2 

Answer 2

Since the library function returns an A, you can't make it return a B without changing it.

One thing you can do is write a function to take the fields of the A instance and copy them over into a new B instance:

class A: # defined by the library     def __init__(self, field):         self.field = field  class B(A): # your fancy new class     def __init__(self, field, field2):         self.field = field         self.field2 = field2 # B has some fancy extra stuff  def b_from_a(a_instance, field2):     """Given an instance of A, return a new instance of B."""     return B(a_instance.field, field2)   a = A("spam") # this could be your A instance from the library b = b_from_a(a, "ham") # make a new B which has the data from a  print b.field, b.field2 # prints "spam ham" 

Edit: depending on your situation, composition instead of inheritance could be a good bet; that is your B class could just contain an instance of A instead of inheriting:

class B2: # doesn't have to inherit from A     def __init__(self, a, field2):         self._a = a # using composition instead         self.field2 = field2      @property     def field(self): # pass accesses to a         return self._a.field     # could provide setter, deleter, etc  a = A("spam") b = B2(a, "ham")  print b.field, b.field2 # prints "spam ham"