How to handle undefined parameteres without decorators

main.py

class UnDef():
    pass

UNDEF = UnDef()

class Block(object):

    def __getattribute__(self, name):
        value = object.__getattribute__(self, name)
        if value == UNDEF:
            raise Exception("Not initialized!")
        return value

class Linear(Block):

    def __init__(self, input_dim=UNDEF):
        self.input_dim = input_dim

    def initialize(self):
        print self.input_dim

b = Linear()
b.initialize() # raises "Not initialized" exception