#classb.py

class classb:

    def foo(self):
        print("this is classb")
    def bar(self,i):
        print("classb:%s" %i)

#classa.py

class classa:

    def foo(self):
        print("this is foo")
    def bar(self,i):
        print("sssss:%s" %i)

#main.py

class Main:
    def __init__(self, module_name):
        self.module_name = module_name
        self.module = None

    def __getattr__(self, funcname):
        if self.module is None:
            self.module = __import__(self.module_name)
            class_tmp=getattr(self.module, self.module_name)
            class_obj = class_tmp()
            func_tmp = getattr(class_obj, funcname)
        return func_tmp

abc = Main('classa')
abc.bar("aaaaa")

abc = Main('classb')
abc.bar("aaaaa")