Prevent creating new attributes outside __init__

Question

I want to be able to create a class (in Python) that once initialized with __init__, does not accept new attributes, but accepts modifications of existing attri

Answer 1

I like very much the solution that uses a decorator, because it's easy to use it for many classes across a project, with minimum additions for each class. But it doesn't work well with inheritance. So here is my version: It only overrides the __setattr__ function - if the attribute doesn't exist and the caller function is not __init__, it prints an error message.

import inspect                                                                                                                             

def froze_it(cls):                                                                                                                      

    def frozensetattr(self, key, value):                                                                                                   
        if not hasattr(self, key) and inspect.stack()[1][3] != "__init__":                                                                 
            print("Class {} is frozen. Cannot set {} = {}"                                                                                 
                  .format(cls.__name__, key, value))                                                                                       
        else:                                                                                                                              
            self.__dict__[key] = value                                                                                                     

    cls.__setattr__ = frozensetattr                                                                                                        
    return cls                                                                                                                             

@froze_it                                                                                                                                  
class A:                                                                                                                                   
    def __init__(self):                                                                                                                    
        self._a = 0                                                                                                                        

a = A()                                                                                                                                    
a._a = 1                                                                                                                                   
a._b = 2 # error