Adding base class to existing object in python

python inheritance metaclass monkeypatching
Hernan picture Hernan · Jun 15, 2012 · Viewed 8.7k times · Source

I have several objects of different kinds (different function names, different signatures) and I monkey patch them to have a common way to access them from different functions. Briefly, there is a dispatcher that takes the objects that I want to patch and depending on the object type it calls different patcher. A patcher will add methods to the object:

def patcher_of_some_type(object):

    def target(self, value):
        # do something and call self methods

    object.target = types.MethodType(target, object)

    # many more of these

As the program grows more complicated making a wrapper around the object (or the object class) seems to be better idea. Some patchers share common code or are interrelated. But I do not control the object creation, nor the class creation. I only get the objects. And even if I could do that, I just want to wrap (or patch) certain objects, not all.

One solution might be add a base class to an existing object, but I am not sure how maintainable and safe this is. Is there another solution?

Answer

ecatmur picture ecatmur · Jun 15, 2012

Dynamically modifying an object's type is reasonably safe, as long as the extra base class is compatible (and you'll get an exception if it isn't). The simplest way to add a base class is with the 3-argument type constructor:

cls = object.__class__
object.__class__ = cls.__class__(cls.__name__ + "WithExtraBase", (cls, ExtraBase), {})

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