Immutable metaclass

immutable.py

from collections import namedtuple


def immutable(fields):
    """
    Returns a metaclass that makes fields on your instances immutable, while
    still allowing __init__ to initialize attributes.

    You should be able to enable immutability on most normal classes by just
    adding

        __metaclass__ = immutable(['attribute_a', 'attribute_b'])
    """
    class Immutable(type):
        def __new__(meta, name, bases, dct):
            tuptype = namedtuple(name + "_tuple", fields)
            class Foo(tuptype):
                def __new__(klass):
                    helper = _ConstructionHelper()
                    init_func = klass.__init__.im_func
                    init_func(helper)
                    initial_data = []
                    for field in fields:
                        initial_data.append(getattr(helper, field))
                    del klass.__init__
                    return super(Foo, klass).__new__(klass, *initial_data)
            return super(Immutable, meta).__new__(meta, name, (Foo,) + bases, dct)
            return klass
    return Immutable


class _ConstructionHelper(object):
    """
    Instances of this are passed to __init__ in immutable classes.
    """


if __name__ == '__main__':
    class MyClass(object):
        __metaclass__ = immutable(["a", "b"])

        def __init__(self):
            self.a = 1
            self.b = 2

    my = MyClass()
    assert my.a == 1
    assert my.b == 2
    try:
        my.a = 2
    except AttributeError:
        print "good: attribute-error on set"