Change python mro at runtime

Question

I\'ve found myself in an unusual situation where I need to change the MRO of a class at runtime.

The code:

class A(object):
    def __init__(self):
          


        

Answer 1

I don't know if it's relevant to the specific problem, but it seems to me that changing the MRO on the fly like that could be risky in a concurrent program, and could definitely have issues if any of these objects turn out to be created recursively.

A non-MRO-based solution occurs to me, depending on the nature of the errors this code would have encountered. (Keeping in mind that this is belated. Perhaps somebody else will want a different answer.)

Basically, each hello() method on B would be wrapped in a decorator. Something along the lines of

class deferring(object):

    def __init__(self, function):
        self.function = function

    def __get__(self, instance, owner):
        # Return an unbound method, or whatever, when called from B.
        if instance is None:
            return self.function.__get__(None, owner)
        else:
            # Indicate that an instance is ready via a flag.
            # May need to tweak this based on the exact problem.
            if hasattr(instance, '_set_up'):
                return self.function.__get__(instance, owner)
            else:
                # Walk the mro manually.
                for cls in owner.__mro__:
                    # Crazy inefficient. Possible to mitigate, but risky.
                    for name, attr in vars(cls).items():
                        if attr is self:
                            break
                    else:
                        continue
                    return getattr(super(cls, instance), name)
                else:
                    raise TypeError

If you don't want to go the descriptor route, it's also possible to do something like

def deferring(function):
    def wrapped(self, *args, **kwargs):
        if hasattr(self, '_set_up'):
            return function(self, *args, **kwargs)
        else:
            for cls in type(self).__mro__:
                for name, attr in vars(cls).items():
                    if attr is function:
                        break
                else:
                    continue
                return getattr(super(cls, self), name)(*args, **kwargs)
            else:
                raise TypeError
    return wrapped