Python - Force an error if an undeclared variable is used inside a class

Consider the following Python code:

class a_class:
    defined_var = 1

    def a_function(self):
        self.defined_var = 2
        self.undefined_var = 3 

The possibility to assign a value to a variable (and then implicitly declare it) that I did not declared at the beginning of the class (like I do for undefined_var in the previous example) is creating me several problems, since for huge classes I forget what I define and I what I don't.

I know that this question may sound silly. I've used to develop using C/C++/Java for a long time where the definition of variables in a class is mandatory...

Is there a way to avoid this? I mean, I would like a behavior like in C/C++/Java where I get an error if I use an undefined variable.

By default, python instances have a __dict__ - a dictionary that stores all of the object's attributes. This is what allows them to store arbitrary attributes. You can suppress the creation of the __dict__ by defining __slots__. This is a class-level variable listing the names of all attributes instances of the class can have. If a class has __slots__, trying to assign to an undefined attribute will throw an exception.

Example:

class MyClass:
    __slots__ = ['defined_var']

    def a_function(self):
        self.defined_var = 2

obj = MyClass()
obj.a_function()
obj.undefined_var = 3  # throws AttributeError