python - 使用元类限制属性类型

Question

我正在尝试使用 Python 进行元类编程，我想知道如何使用元类限制属性类型。使用描述符很容易，但是元类呢？

这是一个简短的例子：

>>> 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版本是2.7

Answer 1

只需__setattr__在元类中覆盖并在初始化期间检查每个属性的默认类型：

>>> 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'>

替代（也许更优雅）的方法：

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

行为与以前相同

Answer 2

在元类中覆盖__setattr__。请注意，您必须分别检查初始值（height=0, path = '/tmp'在您的示例中）：

class RestrictAttrs(type):
    def __new__(mcs, name, bases, dct):
        def _checkattr(k, v):
            if k == 'path':
                if not isinstance(v, str):
                    raise TypeError('path must be a str!')

        def _setattr(self, k, v):
            _checkattr(k, v)
            self.__dict__[k] = v

        # Check of initial values (optional)
        for k,v in dct.items():
            _checkattr(k, v)

        res = type.__new__(mcs, name, bases, dct)
        res.__setattr__ = _setattr
        return res

class Image(object):
    __metaclass__ = RestrictAttrs
    path = '/tmp'

i = Image()
i.path = 32