这个类层次结构的MRO对于oldstyle和newstyle是相同的吗？

Question

我正在阅读Python 2.3 MRO一篇文章，其中给出了以下类层次结构：

>>> O = object
>>> class F(O): pass
>>> class E(O): pass
>>> class D(O): pass
>>> class C(D,F): pass
>>> class B(E,D): pass
>>> class A(B,C): pass

                           6
                          ---
Level 3                  | O |
                       /  ---  \
                      /    |    \
                     /     |     \
                    /      |      \
                  ---     ---    ---
Level 2        2 | E | 4 | D |  | F | 5
                  ---     ---    ---
                   \      / \     /
                    \    /   \   /
                     \  /     \ /
                      ---     ---
Level 1            1 | B |   | C | 3
                      ---     ---
                       \       /
                        \     /
                          ---
Level 0                0 | A |
                          ---

下面的一段话让我很困惑：

A lazy programmer can obtain the MRO directly from Python 2.2, since in this case it coincides with the Python 2.3 linearization.

>>> A.mro()
(<class '__main__.A'>, <class '__main__.B'>, <class '__main__.E'>,
<class '__main__.C'>, <class '__main__.D'>, <class '__main__.F'>,
<type 'object'>)

我感到困惑的原因是，在这篇文章的前面，它说：

Classic classes maintain their old method resolution order, depth first and then left to right.

问题：如果旧式MRO是从左到右的深度优先，那么上述类层次结构的旧式MRO应该是不同的，即ABEODOCDOFO。因此，“懒惰的程序员”不能直接从Python2.2获得MRO。是这样吗？你知道吗

例如，在以下代码段上（不要在Python3.x上运行）：

class O: 
    x = 'o'
class F(O): pass
class E(O): pass
class D(O): pass
class C(D,F): 
    x = 'c'
class B(E,D): pass
class A(B,C): pass

print A.x

如果旧样式的MRO与新样式的MRO（即ABEC等）相同，我希望它打印c。但是，它打印o（即，它是ABEO等）。你知道吗

或者在这里：

class O: pass
class F(O): pass
class E(O): pass
class D(O):
    x = 'd'
class C(D,F): 
    x = 'c'
class B(E,D): pass
class A(B,C): pass

print A.x

如果旧样式的MRO与新样式的MRO（即ABEC等）相同，我希望它打印c。但是，它打印d（即，它是abood等）。你知道吗

我是Python新手，所以我想我错过了什么？你知道吗

Answer 1

见the blog post from Guido（2010年6月），特别是这一重要细节：

The computation of the MRO was officially documented as using a depth-first left-to-right traversal of the classes as before. If any class was duplicated in this search, all but the last occurrence would be deleted from the MRO list.

所以你有ABEODOCDOFO，是的，但是那些更早的O被删除了！你知道吗

Guido编写的“官方文档”声明不是来自您问题中链接的2.3MRO页面，而是来自旧的2.2 MRO page，您将发现其中提到：

Using the classic lookup rule, construct the list of classes that would be searched, including duplicates. Now for each class that occurs in the list multiple times, remove all occurrences except for the last.