Method Resolution Order(MRO)

Classes

• Classic classes

  Before Python 2.2

  Can not use super

  In [1]: class A():
     ...:     def __init__(self):
     ...:         self.a = 1
     ...:         
  
  In [2]: class B():
     ...:     def __init__(self):
     ...:         self.b = 2
     ...: 
  
  In [3]: class C(A, B):
     ...:     def __init__(self):
     ...:         super(C, self).__init__()
     ...:         
  
  In [4]: c = C()
  ---------------------------------------------------------------------------
  TypeError                                 Traceback (most recent call last)
  <ipython-input-4-607856e02023> in <module>()
  ----> 1 c = C()
  
  <ipython-input-3-fca959ece800> in __init__(self)
        1 class C(A, B):
        2     def __init__(self):
  ----> 3         super(C, self).__init__()
        4 
  
  TypeError: super() argument 1 must be type, not classobj
  

• New style classes

  Since Python 2.2

  # Classic classes
  class A():
      pass
  
  # New style classes
  class A(object):
      pass
  

MRO

• Old method resolution order

  Before Python 2.3

  depth first and then left to right
  

• C3 method resolution order

  Since Python 2.3

  Can call instance.__mro__ & instance.mro()

  In [1]: class A(object):
     ...:     def __init__(self):
     ...:         self.a = 1
     ...:         
  
  In [2]: class B(object):
     ...:     def __init__(self):
     ...:         self.b = 2
     ...:         
  
  In [3]: class C(A, B):
     ...:     pass
     ...: 
  
  In [4]: C.__mro__
  Out[4]: (__main__.C, __main__.A, __main__.B, object)
  
  In [5]: C.mro()
  Out[5]: [__main__.C, __main__.A, __main__.B, object]
  

Multiple-inheritance

• Classic classes

  • Parent Not Super & Child Not Super

    In [1]: class A():
       ...:     def __init__(self):
       ...:         self.a = 1
       ...:         
    
    In [2]: class B():
       ...:     def __init__(self):
       ...:         self.b = 2
       ...:         
    
    In [3]: class C(A, B):
       ...:     pass
       ...: 
    
    In [4]: c = C()
    
    In [5]: c.a
    Out[5]: 1
    
    In [6]: c.b
    ---------------------------------------------------------------------------
    AttributeError                            Traceback (most recent call last)
    <ipython-input-6-6be2854bea30> in <module>()
    ----> 1 c.b
    
    AttributeError: C instance has no attribute 'b'
    

  • Parent Not Super & Child Not Super

    In [1]: class A():
       ...:     def __init__(self):
       ...:         self.a = 1
       ...:         
    
    In [2]: class B():
       ...:     def __init__(self):
       ...:         self.b = 2
       ...:         
    
     In [3]: class C(A, B):
       ...:     def __init__(self):
       ...:         A.__init__(self)
       ...:         B.__init__(self)
       ...:
    
    In [4]: c = C()
    
    In [5]: c.a
    Out[5]: 1
    
    In [6]: c.b
    Out[6]: 2
    

• New style classes

  • Parent Not Super

    In [1]: class A(object):
       ...:     def __init__(self):
       ...:         self.a = 1
       ...:         
    
    In [2]: class B(object):
       ...:     def __init__(self):
       ...:         self.b = 2
       ...:         
    
    In [3]: class C(A, B):
       ...:     pass
       ...: 
    # In [3]: class C(A, B):
    #   ...:     def __init__(self):
    #   ...:         super(C, self).__init__()
    #   ...:
    
    In [4]: c = C()
    
    In [5]: c.a
    Out[5]: 1
    
    In [6]: c.b
    ---------------------------------------------------------------------------
    AttributeError                            Traceback (most recent call last)
    <ipython-input-6-6be2854bea30> in <module>()
    ----> 1 c.b
    
    AttributeError: 'C' object has no attribute 'b'
    

  • One Parent Super

    In [1]: class A(object):
       ...:     def __init__(self):
       ...:         self.a = 1
       ...:         super(A, self).__init__()
       ...:         
    
    In [2]: class B(object):
       ...:     def __init__(self):
       ...:         self.b = 2
       ...:         
    
    In [3]: class C(A, B):
       ...:     def __init__(self):
       ...:         super(C, self).__init__()
       ...:         
    
    In [4]: c = C()
    
    In [5]: c.a
    Out[5]: 1
    
    In [6]: c.b
    Out[6]: 2
    
    In [7]: class D(B, A):
       ....:     def __init__(self):
       ....:         super(D, self).__init__()
       ....:         
    
    In [8]: d = D()
    
    In [9]: d.a
    ---------------------------------------------------------------------------
    AttributeError                            Traceback (most recent call last)
    <ipython-input-12-769f1638fd85> in <module>()
    ----> 1 d.a
    
    AttributeError: 'D' object has no attribute 'a'
    
    In [10]: d.b
    Out[10]: 2
    

Order

class A(object):  
    def __init__(self):  
        super(A, self).__init__()  
        print 'A!'

class B(object):  
    def __init__(self):  
        super(B, self).__init__()  
        print 'B!'

class AB(A, B):  
    def __init__(self):  
        super(AB, self).__init__()  
        print 'AB!'

class C(object):  
    def __init__(self):  
        super(C,  self).__init__()  
        print 'C!'

class D(object):  
    def __init__(self):  
        super(D, self).__init__()  
        print 'D!'

class CD(C, D):  
    def __init__(self):  
        super(CD, self).__init__()  
        print 'CD!'

class ABCD(AB, CD):  
    def __init__(self):  
        super(ABCD, self).__init__()  
        print 'ABCD!'

ABCD()

# D!
# C!
# CD!
# B!
# A!
# AB!
# ABCD!

References

[1] Docs@Python, 9. Classes - 9.5.1. Multiple Inheritance

[2] Releases@Python, The Python 2.3 Method Resolution Order

[3] zjm1126@StackOverflow, What does “mro()” do in Python?