什么是甲状腺?它们用于什么?
当前回答
# define a class
class SomeClass(object):
# ...
# some definition here ...
# ...
# create an instance of it
instance = SomeClass()
# then call the object as if it's a function
result = instance('foo', 'bar')
class SomeClass(object):
# ...
# some definition here ...
# ...
def __call__(self, foo, bar):
return bar + foo
但是,正如我们从以前的答案中看到的那样,一个类本身就是一个金属类的例子,所以当我们使用这个类作为一个金属类(即当我们创建一个例子时),我们实际上称它为金属类的 __call__() 方法。
class Meta_1(type):
def __call__(cls):
print "Meta_1.__call__() before creating an instance of ", cls
instance = super(Meta_1, cls).__call__()
print "Meta_1.__call__() about to return instance."
return instance
这是一个使用这个MetaClass的班级。
class Class_1(object):
__metaclass__ = Meta_1
def __new__(cls):
print "Class_1.__new__() before creating an instance."
instance = super(Class_1, cls).__new__(cls)
print "Class_1.__new__() about to return instance."
return instance
def __init__(self):
print "entering Class_1.__init__() for instance initialization."
super(Class_1,self).__init__()
print "exiting Class_1.__init__()."
现在,让我们创建一个类_1的例子。
instance = Class_1()
# Meta_1.__call__() before creating an instance of <class '__main__.Class_1'>.
# Class_1.__new__() before creating an instance.
# Class_1.__new__() about to return instance.
# entering Class_1.__init__() for instance initialization.
# exiting Class_1.__init__().
# Meta_1.__call__() about to return instance.
class type:
def __call__(cls, *args, **kwarg):
# ... maybe a few things done to cls here
# then we call __new__() on the class to create an instance
instance = cls.__new__(cls, *args, **kwargs)
# ... maybe a few things done to the instance here
# then we initialize the instance with its __init__() method
instance.__init__(*args, **kwargs)
# ... maybe a few more things done to instance here
# then we return it
return instance
从上述情况下,它表明,MetaClass的 __call__() 还有机会决定是否会最终对 Class_1.__new__() 或 Class_1.__init__() 进行呼叫。在执行过程中,它实际上可以返回没有被这些方法触摸的对象。
class Meta_2(type):
singletons = {}
def __call__(cls, *args, **kwargs):
if cls in Meta_2.singletons:
# we return the only instance and skip a call to __new__()
# and __init__()
print ("{} singleton returning from Meta_2.__call__(), "
"skipping creation of new instance.".format(cls))
return Meta_2.singletons[cls]
# else if the singleton isn't present we proceed as usual
print "Meta_2.__call__() before creating an instance."
instance = super(Meta_2, cls).__call__(*args, **kwargs)
Meta_2.singletons[cls] = instance
print "Meta_2.__call__() returning new instance."
return instance
class Class_2(object):
__metaclass__ = Meta_2
def __new__(cls, *args, **kwargs):
print "Class_2.__new__() before creating instance."
instance = super(Class_2, cls).__new__(cls)
print "Class_2.__new__() returning instance."
return instance
def __init__(self, *args, **kwargs):
print "entering Class_2.__init__() for initialization."
super(Class_2, self).__init__()
print "exiting Class_2.__init__()."
让我们来看看在重复试图创建类型Class_2的对象时会发生什么。
a = Class_2()
# Meta_2.__call__() before creating an instance.
# Class_2.__new__() before creating instance.
# Class_2.__new__() returning instance.
# entering Class_2.__init__() for initialization.
# exiting Class_2.__init__().
# Meta_2.__call__() returning new instance.
b = Class_2()
# <class '__main__.Class_2'> singleton returning from Meta_2.__call__(), skipping creation of new instance.
c = Class_2()
# <class '__main__.Class_2'> singleton returning from Meta_2.__call__(), skipping creation of new instance.
a is b is c # True
其他回答
类,在Python,是一个对象,和任何其他对象一样,它是一个例子“什么”。这个“什么”是所谓的MetaClass。这个MetaClass是一个特殊类型的类,创造了其他类的对象。因此,MetaClass负责创造新的类。
Class Name Tuple 具有由 Class A 继承的基类 词典具有所有类方法和类变量
另一种方式创建一个金属类是“金属类”的关键词,将金属类定义为一个简单的类,在继承类的参数中,通过金属类=金属类_名称。
Metaclass 可以在以下情况下具体使用:
一个用途是自动将新属性和方法添加到一个例子。
例如,如果你看 Django 模型,它们的定义看起来有点困惑。
class Person(models.Model):
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=30)
然而,在工作时间里,人体对象充满了各种有用的方法。
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。
除了发布的答案,我可以说,一个甲状腺可以定义一个类的行为,所以,你可以明确设置你的甲状腺,每当Python获得一个关键词类,然后它开始搜索甲状腺,如果它没有找到 - 默认甲状腺类型用于创建一个类的对象,使用 __metaclass__属性,你可以设置你的甲状腺类:
class MyClass:
__metaclass__ = type
# write here other method
# write here one more method
print(MyClass.__metaclass__)
它将产生这样的产量:
class 'type'
当然,你可以创建自己的金属类来定义使用你的类创建的任何类的行为。
要做到这一点,您的默认金属类型类必须继承,因为这是主要金属类:
class MyMetaClass(type):
__metaclass__ = type
# you can write here any behaviour you want
class MyTestClass:
__metaclass__ = MyMetaClass
Obj = MyTestClass()
print(Obj.__metaclass__)
print(MyMetaClass.__metaclass__)
产量将是:
class '__main__.MyMetaClass'
class 'type'
看这:
Python 3.10.0rc2 (tags/v3.10.0rc2:839d789, Sep 7 2021, 18:51:45) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> class Object:
... pass
...
>>> class Meta(type):
... test = 'Worked!!!'
... def __repr__(self):
... return 'This is "Meta" metaclass'
...
>>> class ObjectWithMetaClass(metaclass=Meta):
... pass
...
>>> Object or type(Object())
<class '__main__.Object'>
>>> ObjectWithMetaClass or type(ObjectWithMetaClass())
This is "Meta" metaclass
>>> Object.test
AttributeError: ...
>>> ObjectWithMetaClass.test
'Worked!!!'
>>> type(Object)
<class 'type'>
>>> type(ObjectWithMetaClass)
<class '__main__.Meta'>
>>> type(type(ObjectWithMetaClass))
<class 'type'>
>>> Object.__bases__
(<class 'object'>,)
>>> ObjectWithMetaClass.__bases__
(<class 'object'>,)
>>> type(ObjectWithMetaClass).__bases__
(<class 'type'>,)
>>> Object.__mro__
(<class '__main__.Object'>, <class 'object'>)
>>> ObjectWithMetaClass.__mro__
(This is "Meta" metaclass, <class 'object'>)
>>>
换句话说,当一个对象没有创建(对象类型),我们正在寻找MetaClass。
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