Prevent creating new attributes outside __init__

Slots is the way to go:

The pythonic way is to use slots instead of playing around with the __setter__. While it may solve the problem, it does not give any performance improvement. The attributes of objects are stored in a dictionary "__dict__", this is the reason, why you can dynamically add attributes to objects of classes that we have created so far. Using a dictionary for attribute storage is very convenient, but it can mean a waste of space for objects, which have only a small amount of instance variables.

Slots are a nice way to work around this space consumption problem. Instead of having a dynamic dict that allows adding attributes to objects dynamically, slots provide a static structure which prohibits additions after the creation of an instance.

When we design a class, we can use slots to prevent the dynamic creation of attributes. To define slots, you have to define a list with the name __slots__. The list has to contain all the attributes, you want to use. We demonstrate this in the following class, in which the slots list contains only the name for an attribute "val".

class S(object):

    __slots__ = ['val']

    def __init__(self, v):
        self.val = v


x = S(42)
print(x.val)

x.new = "not possible"

=> It fails to create an attribute "new":

42 
Traceback (most recent call last):
  File "slots_ex.py", line 12, in <module>
    x.new = "not possible"
AttributeError: 'S' object has no attribute 'new'

Notes:

1. Since Python 3.3 the advantage optimizing the space consumption is not as impressive any more. With Python 3.3 Key-Sharing Dictionaries are used for the storage of objects. The attributes of the instances are capable of sharing part of their internal storage between each other, i.e. the part which stores the keys and their corresponding hashes. This helps to reduce the memory consumption of programs, which create many instances of non-builtin types. But still is the way to go to avoid dynamically created attributes.

2. Using slots come also with it's own cost. It will break serialization (e.g. pickle). It will also break multiple inheritance. A class can't inherit from more than one class that either defines slots or has an instance layout defined in C code (like list, tuple or int).

I wouldn't use __dict__ directly, but you can add a function to explicitly "freeze" a instance:

class FrozenClass(object):
    __isfrozen = False
    def __setattr__(self, key, value):
        if self.__isfrozen and not hasattr(self, key):
            raise TypeError( "%r is a frozen class" % self )
        object.__setattr__(self, key, value)

    def _freeze(self):
        self.__isfrozen = True

class Test(FrozenClass):
    def __init__(self):
        self.x = 42#
        self.y = 2**3

        self._freeze() # no new attributes after this point.

a,b = Test(), Test()
a.x = 10
b.z = 10 # fails