“元类分辨率顺序”中可能存在的错误？ [英] Possible bug in "metaclass resolution order" ?
问题描述
嗨
我有一个A类，有元类M_A，B类是A的子类，有
元类M_B，M_A的子类。
一个C类，B的子类必须有M_B或它的子类作为元类，
但是如果我需要在C上禁用M_B中的代码吗？正确的方式
这样做似乎是M_C元类，M_B的子类，
实现但不调用父类方法，或调用''type''
方法。
但是如果我尝试使用与M_B无关的其他元类来做到这一点，我会得到一个
" TypeError：元类冲突异常正如预期的那样。
Python 2.4.1（＃1，2005年9月16日，17：47：47）
[GCC 3.3.4] on linux2
输入help，copyright，credit等等。或许可证或更多信息。
Hi
I have a class A, with metaclass M_A, and class B, subclass of A, with
metaclass M_B, subclass of M_A.
A class C, subclass of B must have M_B or a subclass of it as metaclass,
but what if I need to ''disable'' the code in M_B on C ? The correct way
to do that seems to be with a M_C metaclass, subclass of M_B,
implementing but not calling parent class methods, or calling ''type''
methods.
But if I try to do that using other metaclass, not related to M_B, I get a
"TypeError: metaclass conflict exception" as expected.
Python 2.4.1 (#1, Sep 16 2005, 17:47:47)
[GCC 3.3.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
class M_A（type）：pass
.... class A：__ metaclass__ = M_A
....类M_B（M_A）：传递
....类B（A）：__ metaclass__ = M_B
....类M_C（类型）：传递
...... C类（B）：__ metaclass__ = M_C
....
回溯（最近一次调用最后一次）：
文件"< stdin>"，第1行，在？
TypeError：调用元类库时出错
元类冲突：派生类的元类必须是
（非严格）其所有基础元类的子类
问题是，如果我尝试用''type'做同样的事情'
口译员使用M_B而且我没有得到例外，警告或其他任何东西
其他。实际上，C类的__metaclass__属性指向
''类型''但是__class__到M_B！
类C（B）：__ metaclass__ =类型
.... C .__ metaclass__
< type''type''> C .__ class__
< class''__ main __。M_B''> type（C）
class M_A(type): pass .... class A: __metaclass__ = M_A .... class M_B(M_A): pass .... class B(A): __metaclass__ = M_B .... class M_C(type): pass .... class C(B): __metaclass__ = M_C ....
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: Error when calling the metaclass bases
metaclass conflict: the metaclass of a derived class must be a
(non-strict) subclass of the metaclasses of all its bases
The problem is, if I try to do the same thing with ''type'' the
interpreter use M_B and I don''t get an exception, warning or anything
else. In fact, the __metaclass__ attribute of the C class points to
''type'' but the __class__ to M_B!
class C(B): __metaclass__ = type .... C.__metaclass__ <type ''type''> C.__class__ <class ''__main__.M_B''> type(C)
< class''__ main __。M_B''>
由于显式__metaclass__属性有优先于父母
类，这样的情况是一个错误，应该引发一个例外，比如
元类冲突，对吧？
问候，
-
Pedro Werneck
<class ''__main__.M_B''>
Since the explicit __metaclass__ attribute has priority over parent
classes, a case like this is an error and should raise an exception like
the metaclass conflict, right ?
Regards,
--
Pedro Werneck
推荐答案
有你读过Metaclasses和Metaclasses。 在
Python 2.2中统一类型和类的一部分？ （ http://www.python.org/2.2.3 /descrintro.html#metaclasses ）
它讨论并解释了您似乎遇到的问题。
Have you read the "Metaclasses" part of "Unifying types and classes in
Python 2.2"? (http://www.python.org/2.2.3/descrintro.html#metaclasses)
It discusses and explains the issues you seem to have.
2005年9月17日02:04:39 -0700
" Simon Percivall" < PE ******* @ gmail.com>写道：
On 17 Sep 2005 02:04:39 -0700
"Simon Percivall" <pe*******@gmail.com> wrote:
你读过Metaclasses吗？ 在Python 2.2中统一类型和类的一部分？ （ http://www.python.org/2.2.3 /descrintro.html#metaclasses ）
Have you read the "Metaclasses" part of "Unifying types and classes in
Python 2.2"? (http://www.python.org/2.2.3/descrintro.html#metaclasses)
是的，我读过。你读过并理解我的信息吗？ :)
类A的子类，类A的子类，带有元类M_A应该有M_A或
它的子类作为元类。如果你尝试其他任何东西，你会得到一个
TypeError异常。好。但是如果你尝试使用''type''，就不会发生
。
Python 2.4.1（＃1，2005年9月16日，17：47： 47）
[GCC 3.3.4] on linux2
输入help，copyright，credit等等。或许可证或更多信息。
Yes, I read. Have you read and understood my message ? :)
A class B, subclass of class A, with a metaclass M_A should have M_A or
a subclass of it as metaclass. If you try with anything else, you get a
TypeError exception as expected. OK. But if you try with ''type'', nothing
happens.
Python 2.4.1 (#1, Sep 16 2005, 17:47:47)
[GCC 3.3.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
class M_A（type）：pass
.... class A：__ metaclass__ = M_A
.... B类（A）：__ metaclass__ =类型
.... B .__ class__
< class''__ main __。M_A''> B .__元类__
class M_A(type): pass .... class A: __metaclass__ = M_A .... class B(A): __metaclass__ = type .... B.__class__ <class ''__main__.M_A''> B.__metaclass__
< type''type''>
问候，
-
Pedro Werneck
<type ''type''>
Regards,
--
Pedro Werneck
我认为这更像是一个文档问题，而不是一个bug。
一开始可能看起来很奇怪，但是__metaclass__和__class__可能与
不同。
例如，如果M是元类
I think this is more of a documentation issue than of a bug.
It may seems strange at first, but __metaclass__ and __class__ may be
different.
For instance, if M is metaclass
class C（object）：pass
C .__ metaclass__ = M
class C(object): pass C.__metaclass__ = M
你得到一个metaclass hook等于M的类，但C .__ class__是
仍然''type''。
在您的示例中，将__metaclass__设置为''type''不会更改
的元类
创建的类，它继承自基类。我建议您提交文件
bug。遗憾的是，有关元类的基本文档有点缺少b / b
，你可以通过试用和错误发现很多东西。
Michele Simionato
you get a class with metaclass hook equal to M, but C.__class__ is
still ''type''.
In you example, setting the __metaclass__ to ''type'' does not change the
metaclass of
the created class, which is inherited from the base class. I suggest
you to file a documentation
bug. Unfortunately the basic documentation about metaclasses is a bit
lacking and you have
to discover many things by trial and errors.
Michele Simionato
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