How to write metaclass which would prevent creating new attributes after __init__()?

Question

Currently I override the class' __setattr__() towards the end of the class' __init__() method to prevent new attribute creation -

class Point(object):
    def __init__(self):
        self.x = 0
        self.y = 0
        Point.__setattr__ = self._setattr

    def _setattr(self, name, value):
        if not hasattr(self, name):
            raise AttributeError("'" + name + "' not an attribute of Point object.")
        else:
            super(Point, self).__setattr__(name, value)

Is there a way to avoid manually overriding __setattr__() and do this automatically with the help of metaclasses?

The closest I came was -

class attr_block_meta(type):
    def __new__(meta, cname, bases, dctry):
        def _setattr(self, name, value):
            if not hasattr(self, name):
                raise AttributeError("'" + name + "' not an attribute of " + cname + " object.")
            object.__setattr__(self, name, value)

        dctry.update({'x': 0, 'y': 0})
        cls = type.__new__(meta, cname, bases, dctry)
        cls.__setattr__ = _setattr
        return cls

class ImPoint(object):
    __metaclass__ = attr_block_meta

Is there a more generic way of doing this such that apriori knowledge of the subclass attributes is not required?
Basically, how to avoid the line dctry.update({'x': 0, 'y': 0}) and make this work irrespective of what the names of class attributes are?

P.S. - FWIW I have already evaluated the __slots__ and namedtuple options and found them lacking for my needs. Please don't narrow your focus to the pared down Points() example that I have used to illustrate the question; the actual use case involves a far more complex class.

Answer 1

Don't reinvent the wheel.

Two simple ways to achieve that (without directly using a metaclass) are using:

1. namedtuples
2. __slots__

For example, using namedtuple (based on the example in the docs):

Point = namedtuple('Point', ['x', 'y'])
p = Point(11, 22)
p.z = 33  # ERROR

For example, using __slots__:

class Point(object):
    __slots__ = ['x', 'y']
    def __init__(self, x=0, y=0):
        self.x = x
        self.y = y

p = Point(11,22)
p.z = 33  # ERROR