Python: add a parent class to a class after initial evaluation

General Python Question

I'm importing a Python library (call it animals.py) with the following class structure:

class Animal(object): pass

class Rat(Animal): pass

class Bat(Animal): pass

class Cat(Animal): pass

...

I want to add a parent class (Pet) to each of the species classes (Rat, Bat, Cat, ...); however, I cannot change the actual source of the library I'm importing, so it has to be a run time change.

The following seems to work:

import animals

class Pet(object): pass

for klass in (animals.Rat, animals.Bat, animals.Cat, ...): 
    klass.__bases__ = (Pet,) + klass.__bases__

Is this the best way to inject a parent class into an inheritance tree in Python without making modification to the source definition of the class to be modified?

Motivating Circumstances

I'm trying to graft persistence onto the a large library that controls lab equipment. Messing with it is out of the question. I want to give ZODB's Persistent a try. I don't want to write the mixin/facade wrapper library because I'm dealing with 100+ classes and lots of imports in my application code that would need to be updated. I'm testing options by hacking on my entry point only: setting up the DB, patching as shown above (but pulling the species classes w/ introspection on the animals module instead of explicit listing) then closing out the DB as I exit.

Mea Culpa / Request

This is an intentionally general question. I'm interested in different approaches to injecting a parent and comments on the pros and cons of those approaches. I agree that this sort of runtime chicanery would make for really confusing code. If I s开发者_C百科ettle on ZODB I'll do something explicit. For now, as a regular user of python, I'm curious about the general case.

Your method is pretty much how to do it dynamically. The real question is: What does this new parent class add? If you are trying to insert your own methods in a method chain that exists in the classes already there, and they were not written properly, you won't be able to; if you are adding original methods (e.g. an interface layer), then you could possibly just use functions instead.

I am one who embraces Python's dynamic nature, and would have no problem using the code you have presented. Make sure you have good unit tests in place (dynamic or not ;), and that modifying the inheritance tree actually lets you do what you need, and enjoy Python!

You should try really hard not to do this. It is strange, and will likely end in tears.

As @agf mentions, you can use Pet as a mixin. If you tell us more about why you want to insert a parent class, we can help you find a nicer solution.