C3方法解析顺序(The C3 Method Resolution Order)
- 像深度又像广度排序
>>> O = object >>> class F(O): pass >>> class E(O): pass >>> class D(O): pass >>> class C(D,F): pass >>> class B(D,E): pass >>> class A(B,C): pass```sh 6 Level 3 | O | (more general) / --- \ / | \ | / | \ | / | \ | --- --- --- | Level 2 3 | D | 4| E | | F | 5 | --- --- --- | \ \ _ / | | \ / \ _ | | \ / \ | | --- --- | Level 1 1 | B | | C | 2 | --- --- | \ / | \ / \ / --- Level 0 0 | A |L[A] = A + merge(BDEO,CDFO,BC) = A + B + merge(DEO,CDFO,C) = A + B + C + merge(DEO,DFO) = A + B + C + D + merge(EO,FO) = A + B + C + D + E + merge(O,FO) = A + B + C + D + E + F + merge(O,O) = A B C D E F O例2
>>> 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 |
--->>> A.mro()
(<class '__main__.A'>, <class '__main__.B'>, <class '__main__.E'>,
<class '__main__.C'>, <class '__main__.D'>, <class '__main__.F'>,
<type 'object'>)什么时候对MRO来说是非法的？
-
例3
>>> O = object
>>> class X(O): pass
>>> class Y(O): pass
>>> class A(X,Y): pass
>>> class B(Y,X): pass -----------
| |
| O |
| / \ |
- X Y /
| / | /
| / |/
A B
\ /
?L[O] = 0
L[X] = X O
L[Y] = Y O
L[A] = A X Y O
L[B] = B Y X OL[C] = C + merge(AXYO, BYXO, AB)
= C + A + merge(XYO, BYXO, B)
= C + A + B + merge(XYO, YXO)- 这时，我们不能合并列表XYO和列表YXO，：因此，没有了符合规则的head，C3算法停止。在这种情形下，Python 2.3会抛出一个异常(TypeError: MRO conflict among bases Y, X)，因为X在YXO的tail中,而Y在XYO的tail中,阻止创建模棱两可的继承层次。Python 2.2不会抛出异常，而是选择了一种特定的方法解析顺序(CABXYO).
