Python MRO

文中代碼基於Python3.7

 

對於Python中的多繼承情況，運行時在搜索對象的屬性或方法時，需要遵循一定的順序規則，這個規則稱為：Method Resolution Order (MRO).

 

MRO規則可以總結為以下三句話：

• In the multiple inheritance scenario, any specified attribute is searched first in the current class. If not found, the search continues into parent classes in depth-first, left-right fashion without searching the same class twice.

• So, first it goes to super class (and its super classes) given first in the list then second super class (and its super classes) , from left to right order. Then finally Object class, which is a super class for all classes.

  這里的list指的是多個父類組成的list，如：

  class M(X,Y,Z):

  pass

  list就是(X,Y,Z)

• When in MRO we have a super class before subclass then it must be removed from that position in MRO.

  這一句和第一句對應起來看，一個類只被檢索一次，所以基類要往后移

 

可以調用類型對象的mro方法或者__mro__屬性來獲取類型的MRO信息。

 

class X:
    def hello(self):
        print('x')
​
​
class Y:
    def hello(self):
        print('y')
    
    def world(self):
        print('y_world')
​
​
class Z:
    def hello(self):
        print('z')
​
​
class A(X):
    def hello(self):
        print('a')
​
​
class B(Y,Z):
    def hello(self):
        print('b')
​
​
class M(B, A): 
    pass
​
print(M.mro())
print(M.__mro__)
​
# 輸出：
# list類型
[<class '__main__.M'>, <class '__main__.B'>, <class '__main__.Y'>, <class '__main__.Z'>, <class '__main__.A'>, <class '__main__.X'>, <class 'object'>]
# tuple類型
(<class '__main__.M'>, <class '__main__.B'>, <class '__main__.Y'>, <class '__main__.Z'>, <class '__main__.A'>, <class '__main__.X'>, <class 'object'>)

 

MRO圖示如下：

goes to super class (and its super classes) given first in the list then second super class (and its super classes) , from left to right order. Then finally Object class

 

 

depth-first, left-right fashion without searching the same class twice ，得到MRO列表：[<class '__main__.M'>, <class '__main__.B'>, <class '__main__.Y'>, <class '__main__.Z'>, <class '__main__.A'>, <class '__main__.X'>, <class 'object'>]

 

---

 

B和A均繼承自Z，M繼承自B和A：

class X:
    def hello(self):
        print('x')
​
​
class Y:
    def hello(self):
        print('y')
    
    def world(self):
        print('y_world')
​
​
class Z:
    def hello(self):
        print('z')
​
​
class A(X,Z):
    def hello(self):
        print('a')
​
​
class B(Y,Z):
    def hello(self):
        print('b')
​
​
class M(B, A): 
    pass
​
print(M.mro())
​
# 輸出：
# [<class '__main__.M'>, <class '__main__.B'>, <class '__main__.Y'>, <class '__main__.A'>, <class '__main__.X'>, <class '__main__.Z'>, <class 'object'>]

 

MRO圖示如下：

goes to super class (and its super classes) given first in the list then second super class (and its super classes) , from left to right order. Then finally Object class

 

 

depth-first, left-right fashion without searching the same class twice ，得到MRO列表：[<class '__main__.M'>, <class '__main__.B'>, <class '__main__.Y'>, <class '__main__.A'>, <class '__main__.X'>, <class '__main__.Z'>, <class 'object'>]

 

---

 

class X:
    def hello(self):
        print('x')
​
​
class Y:
    def hello(self):
        print('y')
    
    def world(self):
        print('y_world')
​
​
class Z:
    def hello(self):
        print('z')
​
​
class A(X,Z):
    def hello(self):
        print('a')
​
​
class B(Y,Z):
    def hello(self):
        print('b')
​
​
class M(B, A, Y): 
    pass
​
print(M.mro())
​
# 輸出
# [<class '__main__.M'>, <class '__main__.B'>, <class '__main__.A'>, <class '__main__.Y'>, <class '__main__.X'>, <class '__main__.Z'>, <class 'object'>]

 

MRO圖示如下：

goes to super class (and its super classes) given first in the list then second super class (and its super classes) , from left to right order. Then finally Object class

 

 

這個MRO圖可以繼續簡化：

depth-first, left-right fashion without searching the same class twice

 

 

 

得到MRO列表為[<class '__main__.M'>, <class '__main__.B'>, <class '__main__.A'>, <class '__main__.Y'>, <class '__main__.X'>, <class '__main__.Z'>, <class 'object'>]

 

---

 

When in MRO we have a super class before subclass then it must be removed from that position in MRO

 

下面是一個會報錯的示例：

class A:
    def process(self):
        print('A process()')
​
​
class B(A):
    def process(self):
        print('B process()')
​
​
class M(A, B):
    pass
​
print(M.mro())
​
# 輸出：
# TypeError: Cannot create a consistent method resolution
# order (MRO) for bases A, B

 

MRO圖示：

 

 

如果一個方法或屬性同時存在與B和A，應為M直接繼承B又直接繼承A，那么通過M來調用時就不知道是該從B中還是A中獲取這個方法或屬性了，干脆就報錯吧。我覺得MRO順序應該為：M->B->A->object。

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