Python Classes Inheritance

Question

I am having a bit trouble understanding python's mro(Method resolution order). Python operates from left to right. So as per my understanding the order of calling for M should be. M, B, A, Z, X, Y

class X: pass
class Y: pass
class Z: pass

class A(X,Y): pass
class B(Y,Z): pass

class M(B,A,Z): pass

print(M.mro())

But the correct order of calling is.

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

Can anyone explain the ordering and help me understand this. Thank you

Python operates from left to right: no, it does not. It implements C3 linearization algorithm — Łukasz Rogalski
– Łukasz Rogalski, Commented Jun 2, 2016 at 7:14

coredump · Accepted Answer · 2016-06-02 07:20:30Z

mro has been based on C3 algorithm since Python2.3, and 'merge' is core of that algorithm. how merge works. According to your code, here we can list how to get mro(M):

mro(M) = [M] + merge(mro(B), mro(A), mro(Z), [B,A,Z])    

mro(B) = [B] + merge(mro(Y) , mro(Z)) = [B] + merge([Y] , [Z])
       = [B,Y] + merge([Z]) 
       = [B,Y,Z]

mro(A) = [A] + merge(mro(X) , mro(Y)) 
       = [A] + merge([X],[Y]) 
       = [A,X,Y]

mro(Z) = [Z]

Back to mro(M):

mro(M) = [M] + merge([B,Y,Z), [A,X,Y], [Z], [B,A,Z]) 
       = [M,B] + merge([Y,Z], [A,X,Y], [Z], [A,Z]) 
       = [M,B,A] + merge([Y,Z], [X,Y], [Z], [Z]) 
       = [M,B,A,X] + merge([Y,Z] + [Y] + [Z], [Z]) 
       = [M,B,A,X,Y] + merge([Z] + [Z] + [Z]) 
       = [M,B,A,X,Y,Z]

Collectives™ on Stack Overflow

Python Classes Inheritance

1 Answer 1

Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Related