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

For your particular example, one solution is to give B a class attribute to hold a default message:

class B(A):

    msg_str = "default msg"

    def __init__(self):
        super(B, self).__init__()
        print "__init__ B"
        self.msg_str = "B"
        self.hello()

    def hello(self):
        print "%s hello" % self.msg_str

Usually this causes confusion, but in this case it might be helpful. If B.hello is called before the instance's msg_str is set, it will read the class one. Once the instance msg_str is set, it shadows the class one so that future accesses of self.msg_str will see the instance-specific one.

I don't quite understand why you can't set attributes before calling the superclass __init__. Depending on exactly what the underlying situation is, there may be other solutions.

Answer 2

Call self.msg_str = "B" before super(B, self).__init__().