python mro文件_Python MRO
文中代碼基于Python3.7
對(duì)于Python中的多繼承情況，運(yùn)行時(shí)在搜索對(duì)象的屬性或方法時(shí)，需要遵循一定的順序規(guī)則，這個(gè)規(guī)則稱(chēng)為：Method Resolution Order (MRO).
MRO規(guī)則可以總結(jié)為以下三句話(huà)：
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(gè)父類(lèi)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(duì)應(yīng)起來(lái)看，一個(gè)類(lèi)只被檢索一次，所以基類(lèi)要往后移
可以調(diào)用類(lèi)型對(duì)象的mro方法或者_(dá)_mro__屬性來(lái)獲取類(lèi)型的MRO信息。
classX:defhello(self):print('x')
?
?classY:defhello(self):print('y')defworld(self):print('y_world')
?
?classZ:defhello(self):print('z')
?
?classA(X):defhello(self):print('a')
?
?classB(Y,Z):defhello(self):print('b')
?
?classM(B, A):pass?print(M.mro())print(M.__mro__)
?#輸出：#list類(lèi)型
[, , , , , , ]#tuple類(lèi)型
(, , , , , , )
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：[, , , , , , ]
B和A均繼承自Z，M繼承自B和A：
classX:defhello(self):print('x')
?
?classY:defhello(self):print('y')defworld(self):print('y_world')
?
?classZ:defhello(self):print('z')
?
?classA(X,Z):defhello(self):print('a')
?
?classB(Y,Z):defhello(self):print('b')
?
?classM(B, A):pass?print(M.mro())
?#輸出：#[, , , , , , ]
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：[, , , , , , ]
classX:defhello(self):print('x')
?
?classY:defhello(self):print('y')defworld(self):print('y_world')
?
?classZ:defhello(self):print('z')
?
?classA(X,Z):defhello(self):print('a')
?
?classB(Y,Z):defhello(self):print('b')
?
?classM(B, A, Y):pass?print(M.mro())
?#輸出#[, , , , , , ]
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(gè)MRO圖可以繼續(xù)簡(jiǎn)化：
depth-first, left-right fashion without searching the same class twice
得到MRO列表為[, , , , , , ]
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(gè)會(huì)報(bào)錯(cuò)的示例：
classA:defprocess(self):print('A process()')
?
?classB(A):defprocess(self):print('B process()')
?
?classM(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(gè)方法或?qū)傩酝瑫r(shí)存在與B和A，應(yīng)為M直接繼承B又直接繼承A，那么通過(guò)M來(lái)調(diào)用時(shí)就不知道是該從B中還是A中獲取這個(gè)方法或?qū)傩粤?#xff0c;干脆就報(bào)錯(cuò)吧。我覺(jué)得MRO順序應(yīng)該為：M->B->A->object。
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