Restricting attribute type with metaclass

metaclass, python

Solution

Just override `__setattr__` in the metaclass and check default type for every attribute during initialization:

>>> class Meta(type):
    def __new__(meta, name, bases, dict):
        def _check(self, attr, value):
            if attr in self.defaults:
                if not isinstance(value, self.defaults[attr]):
                    raise TypeError('%s cannot be %s' % (attr, type(value)))
            else:                        
                self.defaults[attr] = type(value)

        def _setattr(self, attr, value):
            _check(self, attr, value)
            object.__setattr__(self, attr, value)

        cls = type.__new__(meta, name, bases, dict)
        # Set up default type for every attribute
        cls.defaults = {name: type(value) for name, value in dict.items()}
        cls.__setattr__ = _setattr
        return cls


>>> class Image(object):
    __metaclass__ = Meta
    height = 0
    width = 0
    path = '/tmp'
    size = 0


>>> i = Image()
>>> i.height = 240
>>> i.height
240
>>> i.size
0
>>> i.size = 7
>>> i.size
7
>>> i.path = '/tmp/subdir'
>>> i.path
'/tmp/subdir'
>>> i.path = 23
TypeError: path cannot be <type 'int'>

Alternative (and maybe more elegant) method:

class MetaBase(object):
    def _check(self, attr, value):
        if attr in self.defaults:
            if not isinstance(value, self.defaults[attr]):
                raise TypeError('%s cannot be %s' % (attr, type(value)))
        else:
            self.defaults[attr] = type(value)
    def __setattr__(self, attr, value):
        self._check(attr, value)
        super(MetaBase, self).__setattr__(attr, value)

class Meta(type):
    def __new__(meta, name, bases, dict):
        cls = type.__new__(meta, name, (MetaBase,) + bases, dict)
        cls.defaults = {name: type(value) for name, value in dict.items()}
        return cls

class Image(object):
    __metaclass__ = Meta
    height = 0
    width = 0
    path = '/tmp'
    size = 0

Behaviour is the same as before

Problem

I'm trying to get into metaclass programming in Python and I'd like to know how to restrict attribute type with metaclass. It's quite easy with the descriptors, but what about metaclasses? Here is short example: ``` >>> class Image(Object): ... height = 0 ... width = 0 ... path = '/tmp' ... size = 0 >>> img = Image() >>> img.height = 340 >>> img.height 340 >>> img.path = '/tmp/x00.jpeg' >>> img.path '/tmp/x00.jpeg' >>> img.path = 320 Traceback (most recent call last): ... TypeError ``` Python version is 2.7

Original source