Python 3 更新

在一个甲状腺中,有(目前)两个关键方法:

__prepare__ 允许您提供自定义地图(如 OrderedDict)作为名称空间使用,而类正在创建。

__new__ 负责最终类的实际创建/修改。

一个色彩色彩,不做任何东西 - 额外的金属类会喜欢:

class Meta(type):

    def __prepare__(metaclass, cls, bases):
        return dict()

    def __new__(metacls, cls, bases, clsdict):
        return super().__new__(metacls, cls, bases, clsdict)

一个简单的例子:

说你想要一些简单的验证代码在你的属性上运行 - 因为它必须总是一个 int 或 str. 没有一个 metaclass,你的类会看起来像:

class Person:
    weight = ValidateType('weight', int)
    age = ValidateType('age', int)
    name = ValidateType('name', str)

正如你可以看到的那样,你必须重复属性的名称两次,这使得类型与刺激的错误一起可能。

一个简单的甲状腺可以解决这个问题:

class Person(metaclass=Validator):
    weight = ValidateType(int)
    age = ValidateType(int)
    name = ValidateType(str)

class Validator(type):
    def __new__(metacls, cls, bases, clsdict):
        # search clsdict looking for ValidateType descriptors
        for name, attr in clsdict.items():
            if isinstance(attr, ValidateType):
                attr.name = name
                attr.attr = '_' + name
        # create final class and return it
        return super().__new__(metacls, cls, bases, clsdict)

一个样本运行:

p = Person()
p.weight = 9
print(p.weight)
p.weight = '9'

生产:

9
Traceback (most recent call last):
  File "simple_meta.py", line 36, in <module>
    p.weight = '9'
  File "simple_meta.py", line 24, in __set__
    (self.name, self.type, value))
TypeError: weight must be of type(s) <class 'int'> (got '9')

注意:这个例子是简单的,它也可能已经完成了一个类装饰师,但假设一个真正的金属玻璃会做得更多。

class ValidateType:
    def __init__(self, type):
        self.name = None  # will be set by metaclass
        self.attr = None  # will be set by metaclass
        self.type = type
    def __get__(self, inst, cls):
        if inst is None:
            return self
        else:
            return inst.__dict__[self.attr]
    def __set__(self, inst, value):
        if not isinstance(value, self.type):
            raise TypeError('%s must be of type(s) %s (got %r)' %
                    (self.name, self.type, value))
        else:
            inst.__dict__[self.attr] = value

Metaclasses 是做“类”的工作的秘密酱油,新风格对象的默认 metaclass 被称为“类型”。

class type(object)
  |  type(object) -> the object's type
  |  type(name, bases, dict) -> a new type

Metaclasses 取 3 args. 'name', 'bases' 和 'dict'

查找这个例子类定义中的名称、基础和字符号来源于哪里。

class ThisIsTheName(Bases, Are, Here):
    All_the_code_here
    def doesIs(create, a):
        dict

def test_metaclass(name, bases, dict):
    print 'The Class Name is', name
    print 'The Class Bases are', bases
    print 'The dict has', len(dict), 'elems, the keys are', dict.keys()

    return "yellow"

class TestName(object, None, int, 1):
    __metaclass__ = test_metaclass
    foo = 1
    def baz(self, arr):
        pass

print 'TestName = ', repr(TestName)

# output => 
The Class Name is TestName
The Class Bases are (<type 'object'>, None, <type 'int'>, 1)
The dict has 4 elems, the keys are ['baz', '__module__', 'foo', '__metaclass__']
TestName =  'yellow'

现在,一个实际上意味着什么的例子,这将自动使列表中的变量“属性”设置在课堂上,并设置为无。

def init_attributes(name, bases, dict):
    if 'attributes' in dict:
        for attr in dict['attributes']:
            dict[attr] = None

    return type(name, bases, dict)

class Initialised(object):
    __metaclass__ = init_attributes
    attributes = ['foo', 'bar', 'baz']

print 'foo =>', Initialised.foo
# output=>
foo => None

请注意,启动者获得的魔法行为是通过拥有金属类的 init_属性而没有转移到启动者的子类。

这里是一个更具体的例子,显示如何可以创建一个在创建一个类时执行一个行动的甲型类的“类型”。

class MetaSingleton(type):
    instance = None
    def __call__(cls, *args, **kw):
        if cls.instance is None:
            cls.instance = super(MetaSingleton, cls).__call__(*args, **kw)
        return cls.instance

class Foo(object):
    __metaclass__ = MetaSingleton

a = Foo()
b = Foo()
assert a is b

下面是另一个例子,它可以用于什么:

您可以使用甲状腺来改变其例子(类)的功能。

class MetaMemberControl(type):
    __slots__ = ()

    @classmethod
    def __prepare__(mcs, f_cls_name, f_cls_parents,  # f_cls means: future class
                    meta_args=None, meta_options=None):  # meta_args and meta_options is not necessarily needed, just so you know.
        f_cls_attr = dict()
        if not "do something or if you want to define your cool stuff of dict...":
            return dict(make_your_special_dict=None)
        else:
            return f_cls_attr

    def __new__(mcs, f_cls_name, f_cls_parents, f_cls_attr,
                meta_args=None, meta_options=None):

        original_getattr = f_cls_attr.get('__getattribute__')
        original_setattr = f_cls_attr.get('__setattr__')

        def init_getattr(self, item):
            if not item.startswith('_'):  # you can set break points at here
                alias_name = '_' + item
                if alias_name in f_cls_attr['__slots__']:
                    item = alias_name
            if original_getattr is not None:
                return original_getattr(self, item)
            else:
                return super(eval(f_cls_name), self).__getattribute__(item)

        def init_setattr(self, key, value):
            if not key.startswith('_') and ('_' + key) in f_cls_attr['__slots__']:
                raise AttributeError(f"you can't modify private members:_{key}")
            if original_setattr is not None:
                original_setattr(self, key, value)
            else:
                super(eval(f_cls_name), self).__setattr__(key, value)

        f_cls_attr['__getattribute__'] = init_getattr
        f_cls_attr['__setattr__'] = init_setattr

        cls = super().__new__(mcs, f_cls_name, f_cls_parents, f_cls_attr)
        return cls


class Human(metaclass=MetaMemberControl):
    __slots__ = ('_age', '_name')

    def __init__(self, name, age):
        self._name = name
        self._age = age

    def __getattribute__(self, item):
        """
        is just for IDE recognize.
        """
        return super().__getattribute__(item)

    """ with MetaMemberControl then you don't have to write as following
    @property
    def name(self):
        return self._name

    @property
    def age(self):
        return self._age
    """


def test_demo():
    human = Human('Carson', 27)
    # human.age = 18  # you can't modify private members:_age  <-- this is defined by yourself.
    # human.k = 18  # 'Human' object has no attribute 'k'  <-- system error.
    age1 = human._age  # It's OK, although the IDE will show some warnings. (Access to a protected member _age of a class)

    age2 = human.age  # It's OK! see below:
    """
    if you do not define `__getattribute__` at the class of Human,
    the IDE will show you: Unresolved attribute reference 'age' for class 'Human'
    but it's ok on running since the MetaMemberControl will help you.
    """


if __name__ == '__main__':
    test_demo()

金星是强大的,有很多事情(如猴子魔法)你可以用它,但要小心,这可能只是你知道的。

我看到一个有趣的使用案例在一个名为类用途的包中,它检查是否所有类变量在顶部案例格式(方便有统一逻辑的配置类),并检查是否没有例子级方法在课堂上。

此分類上一篇: tl;dr version

类型(obj)函数会给你一个对象的类型。

一个阶级的类型( )是它的甲型阶级。

使用甲状腺:

class Foo(object):
    __metaclass__ = MyMetaClass

一个类的类是一个类的类 - 一个类的身体是转移到一个类的论点,它被用来构建一个类。

在这里,你可以阅读如何使用金属玻璃来自定义课堂建筑。

其他人已经解释了金属玻璃是如何工作的,它们是如何适应Python类型系统的,这里有一个例子,它们可以用于什么。在我写的测试框架中,我想跟踪在哪个类被定义的顺序,以便我后来能够在这个顺序中安装它们,我发现使用金属玻璃最容易做到这一点。

class MyMeta(type):

    counter = 0

    def __init__(cls, name, bases, dic):
        type.__init__(cls, name, bases, dic)
        cls._order = MyMeta.counter
        MyMeta.counter += 1

class MyType(object):              # Python 2
    __metaclass__ = MyMeta

class MyType(metaclass=MyMeta):    # Python 3
    pass

任何是 MyType 的子类,然后获得一个类属性 _ 命令,记录了类被定义的顺序。