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

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'