Python Metaclasses

Python Metaclasses

https://realpython.com/python-metaclasses/#type-and-class

Type and Class

type is a metaclass, of which classes are instances. Just as an ordinary object is an instance of a class, any new-style class in Python, and thus any class in Python 3, is an instance of the type metaclass.

In the above case:

• x is an instance of class Foo.
• Foo is an instance of the type metaclass.
• type is also an instance of the type metaclass, so it is an instance of itself.

Defining a Class Dynamically

def f(obj):
    print('attr =', obj.attr)

Foo = type(
    'Foo',
    (),
    {
        'attr': 100,
        'attr_val': f
    }
)

x = Foo()
x.attr

x.attr_val()

等价于

def f(obj):
    print('attr =', obj.attr)

class Foo:
    attr = 100
    attr_val = f


x = Foo()
x.attr

x.attr_val()

Custom Metaclasses

class Meta(type):
    def __new__(cls, name, bases, dct):
        x = super().__new__(cls, name, bases, dct)
        x.attr = 100
        return x


class Foo(metaclass=Meta):
    pass

Foo.attr

等价于

class Meta(type):
    def __new__(cls, name, bases, dct):
        x = super().__new__(cls, name, bases, dct)
        x.attr = 100
        return x


Foo = Meta('Foo', (), {}):

Foo.attr

实例产生过程

class Foo:
    pass

f = Foo()

The expression Foo() creates a new instance of class Foo. When the interpreter encounters Foo(), the following occurs:

• The __call__() method of Foo’s parent class is called. Since Foo is a standard new-style class, its parent class is the type metaclass, so type’s __call__() method is invoked.

• That __call__() method in turn invokes the following:

  • __new__()
  • __init__()

If Foo does not define __new__() and __init__(), default methods are inherited from Foo’s ancestry. But if Foo does define these methods, they override those from the ancestry, which allows for customized behavior when instantiating Foo.

https://blog.ionelmc.ro/2015/02/09/understanding-python-metaclasses/

类是元类的实例

类的实例的产生， 是依赖元类的 __call__ 接口， 并调用类的 __new__ and __init__

To start things off, a diagram of how instances are constructed:

How to read this swim lane diagram:

• The horizontal lanes is the place where you define the functions.
• Solid lines mean a function call.
  • A line from Metaclass.__call__ to Class.__new__ means Metaclass.__call__ will call Class.__new__.
• Dashed lines means something is returned.
  • Class.__new__ returns the instance of Class.
  • Metaclass.__call__ returns whatever Class.__new__ returned (and if it returned an instance of Class it will also call Class.__init__ on it). [16]
• The number in the red circle signifies the call order.

类产生过程

class Meta(type):
    def __new__(cls, name, bases, dct):
        x = super().__new__(cls, name, bases, dct)
        x.attr = 100
        return x


class Foo(metaclass=Meta):
    pass

Foo.attr

等价于

def new(cls, name, bases, dct):

      x = super().__new__(cls, name, bases, dct)
      x.attr = 100
      return x

Meta = type('Meta', (type), {'__new__': new})

Foo = Meta('Foo', (), {})

为了生成Foo实例， 调用Meta类的call接口， Meta没有call接口， 则会向上找到type的call接口， 然后执行实例化过程，

进而调用 Meta的 __new__ 生成一个类，对应变量为x

Note that a custom __new__() method has been defined for Meta. It wasn’t possible to do that to the type metaclass directly. The __new__() method does the following:

• Delegates via super() to the __new__() method of the parent metaclass (type) to actually create a new class
• Assigns the custom attribute attr to the class, with a value of 100
• Returns the newly created class

按照如下文档翻译， Foo类产生的语句为

type(Meta).__call__(Meta, 'Foo', (), {})

https://docs.python.org/3/reference/datamodel.html#object.__call__

object.__call__(self[, args...])¶

Called when the instance is “called” as a function; if this method is defined, x(arg1, arg2, ...) roughly translates to type(x).__call__(x, arg1, ...).

https://blog.ionelmc.ro/2015/02/09/understanding-python-metaclasses/

def new(cls, name, bases, dct):

      x = super().__new__(cls, name, bases, dct)
      x.attr = 100
      return x



Meta = type('Meta', (type), {'__new__': new})

Foo = Meta('Foo', (), {})

结合如上代码，我们来理解下类创建的过程：

meta class == Meta

meta meta class == Type

Meta 类 跟一般类的差别在于， 其继承了Type类， 这样当Meta产生的类，则继承了Type类的__call__ 和 __new__ __init__ 接口，

这样Meta类也就是一个Type类的一个特例， call调用（Meta()）， 产生实例为class， 这个class 也具有的call产生实例的能力。

Creating a class is quite similar:

Few more notes:

• Metaclass.__prepare__ just returns the namespace object (a dictionary-like object as explained before).
• Metaclass.__new__ returns the Class object.
• MetaMetaclass.__call__ returns whatever Metaclass.__new__ returned (and if it returned an instance of Metaclass it will also call Metaclass.__init__ on it). [16]

So you see, metaclasses allow you to customize almost every part of an object life-cycle.

形式化推导过程

参考

https://blog.ionelmc.ro/2015/02/09/understanding-python-metaclasses/

What is a "callable"?

https://stackoverflow.com/questions/111234/what-is-a-callable

可调用对象具有两种条件：

•  类含有__call__方法， 则实例是callable的， 则下面是合法的： instance()， 将调用 __call__
• 被type这个元类 或者 type的子类 生成的 普通类， 则下面是合法的： class()， 将生成实例 instance

A callable is anything that can be called.

The built-in callable (PyCallable_Check in objects.c) checks if the argument is either:

• an instance of a class with a __call__ method or
• is of a type that has a non null tp_call (c struct) member which indicates callability otherwise (such as in functions, methods etc.)

The method named __call__ is (according to the documentation)

Called when the instance is ''called'' as a function

class Foo:
  def __call__(self):
    print 'called'

foo_instance = Foo()
foo_instance() #this is calling the __call__ method