[英]Old-style classes, new-style classes and metaclasses
In Python 2.x, all new-style classes inherit from object
implicitly or explicitly. 在Python 2.x中,所有新式类都隐式或显式地从
object
继承。 Then look at this: 然后看这个:
>>> class M(type):
... pass
...
>>> class A:
... __metaclass__ = M
...
>>> class B:
... pass
...
>>> a = A()
>>> b = B()
>>> type(A)
<class '__main__.M'>
>>> type(a)
<class '__main__.A'>
Does this mean A
is a new-style class? 这是否意味着
A
是一种新型类? But A
doesn't inherit from object
anyway, right? 但是
A
仍然不会从object
继承,对吗?
>>> type(B)
<class 'classobj'>
>>> type(b)
<type 'instance'>
OK, B
is a classic class, isn't it? 好吧,
B
是经典班,不是吗?
>>> isinstance(A, object)
True
>>> isinstance(B, object)
True
why are instances of both A
and B
instances of object
? 为什么
object
A
和B
实例都是实例?
If B
is an instance of object
, then type(B)
wouldn't be classobj
, right? 如果
B
是object
的实例,则type(B)
不会是classobj
,对吧?
About metaclasses you may read here: http://docs.python.org/reference/datamodel.html#customizing-class-creation . 关于元类,您可以在这里阅读: http : //docs.python.org/reference/datamodel.html#customizing-class-creation 。 Generally metaclasses are intended to work with new style classes.
通常,元类旨在与新样式类一起使用。 When you write:
当你写:
class M(type):
pass
and you use: 而您使用:
class C:
__metaclass__ = M
you will create a new style object because the way M is defined (default implementation uses type
to create a new-style class). 您将创建一个新样式对象,因为定义了M的方式(默认实现使用
type
来创建新样式类)。 You could always implement you own metaclass that would create old-style classes using types.ClassType
. 您始终可以实现自己的元类,该元类将使用
types.ClassType
创建老式的类。
About slots you may read here http://docs.python.org/release/2.5.2/ref/slots.html , a fragment: 关于插槽,您可以在此处http://docs.python.org/release/2.5.2/ref/slots.html中阅读片段:
By default, instances of both old and new-style classes have a dictionary for attribute storage.
默认情况下,新旧类的实例都具有用于属性存储的字典。
For new-style classes you may add __slots__
, then the per-object dictionary will not be created. 对于新型类,您可以添加
__slots__
,然后将不会创建每个对象的字典。
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