__metaclass__是否应该在Python中强制使用元类? [英] Shouldn't __metaclass__ force the use of a metaclass in Python?

问题描述

我一直在尝试学习Python中的元类.我掌握了主要思想，但似乎无法激活该机制.据我了解，在构造类K时，可以通过在全局或类级别将__metaclass__设置为M来将M指定为元类.为了对此进行测试，我编写了以下程序:

I've been trying to learn about metaclasses in Python. I get the main idea, but I can't seem to activate the mechanism. As I understand it, you can specify M to be as the metaclass when constructing a class K by setting __metaclass__ to M at the global or class level. To test this out, I wrote the following program:

p = print

class M(type):
    def __init__(*args):
        type.__init__(*args)
        print("The rain in Spain")

p(1)
class ClassMeta:
    __metaclass__ = M

p(2)
__metaclass__ = M
class GlobalMeta: pass

p(3)
M('NotMeta2', (), {})

p(4)

但是，当我运行它时，会得到以下输出:

However, when I run it, I get the following output:

C:\Documents and Settings\Daniel Wong\Desktop>python --version
Python 3.0.1

C:\Documents and Settings\Daniel Wong\Desktop>python meta.py
1
2
3
The rain in Spain
4

我应该在1和2之后看不到西班牙的雨"吗?这是怎么回事?

Shouldn't I see "The rain in Spain" after 1 and 2? What's going on here?

推荐答案

在Python 3(您正在使用)中，元类由类定义中的关键字参数指定:

In Python 3 (which you are using) metaclasses are specified by a keyword parameter in the class definition:

class ClassMeta(metaclass=M):
  pass

指定__metaclass__类属性或全局变量是Python 2.x的旧语法，不再受支持.另请参见"Python 3的新增功能" 和 PEP 2115 .

Specifying a __metaclass__ class property or global variable is old syntax from Python 2.x and not longer supported. See also "What's new in Python 3" and PEP 2115.

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