Python 为什么元类定义改变了经典类的mro？

根据：

经典类使用了一个简单的MRO方案：在查找方法时， 基类是使用从左到右的简单深度优先搜索的 计划

这可以在Python2.6中验证

In [1]: import sys

In [2]: sys.version
Out[2]: '2.6.6 (r266:84292, Jul 23 2015, 15:22:56) \n[GCC 4.4.7 20120313 (Red Hat 4.4.7-11)]'

In [3]: 
class D:
    def f(self): return 'D'
class B(D): pass
class C(D):
    def f(self): return 'C'
class A(B, C): pass
   ...: 

In [4]: A().f()
Out[4]: 'D'

但是，如果我定义了一个元类，我在Python 2.7.12中得到了一个不同的结果：

Python 2.7.12 (default, Nov 19 2016, 06:48:10) 
Type "copyright", "credits" or "license" for more information.

IPython 5.4.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: class D:       # Note: Old-style
   ...:     def f(self): return "D.f()"
   ...: class B(D): pass
   ...: class C(D):
   ...:     def f(self): return "C.f()"
   ...: class A(B, C): pass
   ...: 

In [2]: A().f()
Out[2]: 'D.f()'    # It works as expected.

In [3]: class __metaclass__(type):
   ...:     "All classes are metamagically modified to be nicely printed"
   ...:     __repr__ = lambda cls: cls.__name__
   ...:     

In [4]: class D:       # Note: Old-style
   ...:     def f(self): return "D.f()"
   ...: class B(D): pass
   ...: class C(D):
   ...:     def f(self): return "C.f()"
   ...: class A(B, C): pass
   ...: 

In [5]: A().f()
Out[5]: 'C.f()'   # WTF??

---

元类仅适用于新样式的类，并且通过在类主体中具有名为

\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu。具有
\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu

class J:
  pass
class K(object):
  pass
class L:
  __metaclass__ = type

type(J) # <type 'classobj'>
type(K) # <type 'type'>
type(L) # <type 'type'>

元类仅适用于新样式的类，并且通过在类主体中具有名为
\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu。具有
\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu

class J:
  pass
class K(object):
  pass
class L:
  __metaclass__ = type

type(J) # <type 'classobj'>
type(K) # <type 'type'>
type(L) # <type 'type'>

我无法在
2.7.14[GCC 4.2.1兼容的Apple LLVM 9.0.0（clang-900.0.37）]
（Mac OSX）、
2.7.13（默认，2017年1月19日，14:48:08）\n[GCC 6.3.0 20170118]
（Debian 9.0）或
2.7.12（默认，2016年11月19日，06:48:10）\n[GCC 5.0 20160609]
（Ubuntu 16.04，看起来与您运行的版本完全相同）。结果总是
D.f（）
。只有当我使用新样式的类时，它们才会报告
C.f（）
@birryrree很抱歉我提出了一个误导性的问题。我发现我的结果是由于元类之前的定义。我已经修改了这个问题。我无法在
2.7.14[GCC 4.2.1兼容的Apple LLVM 9.0.0（clang-900.0.37）]
（Mac OSX），
2.7.13（默认，2017年1月19日，14:48:08）中的任何一个上重现你的2.7行为\n[GCC 6.3.0 20170118]
（Debian 9.0）或
2.7.12（默认值，2016年11月19日06:48:10）\n[GCC 5.4.0 20160609]
（Ubuntu 16.04，看起来与您运行的版本完全相同）。结果总是
D.f（）
。只有当我使用新样式的类时，它们才会报告
C.f（）
@birryrree很抱歉我的问题有误导性。我发现我的结果是由于元类的先前定义。我已经修改了这个问题。