frozenobject

frozenobject.py

class FrozenMeta(type):
    """Metaclass for freezing a type after construction.  Only ``__init__``
    will be allowed to assign to attributes.
    """
    def __call__(cls, *args, **kwargs):
        obj = super(FrozenMeta, cls).__call__(*args, **kwargs)
        obj.__frozen__ = True
        return obj


class frozenobject(object, metaclass=FrozenMeta):
    __slots__ = ('__frozen__',)

    def __setattr__(self, attr, value):
        if getattr(self, '__frozen__', False):
            raise FrozenError("This object is frozen and cannot be modified.")

        super(frozenobject, self).__setattr__(attr, value)

    def __delattr__(self, attr):
        if getattr(self, '__frozen__', False):
            raise FrozenError("This object is frozen and cannot be modified.")

        super(frozenobject, self).__delattr__(attr)


class FrozenError(TypeError):
    pass


class MediaItem(frozenobject):
    def __init__(self, type, id, url, caption):
        self.type = type
        self.id = id
        self.url = url
        self.caption = caption

    @property
    def is_video(self):
        return self.type == 'video'


class MediaItem2(frozenobject):
    __slots__ = ('type', 'id', 'url', 'caption')
    def __init__(self, type, id, url, caption):
        self.type = type
        self.id = id
        self.url = url
        self.caption = caption

    @property
    def is_video(self):
        return self.type == 'video'

import pytest

item = MediaItem('video', 3, 'http://.../', 'this is my video')
print(item.is_video)
with pytest.raises(FrozenError):
    item.adhoc = 'lololol'
with pytest.raises(FrozenError):
    item.type = 'photo'


item = MediaItem2('video', 3, 'http://.../', 'this is my video')
print(item.is_video)
with pytest.raises(FrozenError):
    item.adhoc = 'lololol'
with pytest.raises(FrozenError):
    item.type = 'photo'