python中多重继承中的super()用法

Question

I am new to python. I am trying to understand super() functionality in python multiple inheritance.

class B():
    def __init__(self):
        print("__init__ of B called")
        self.b = "B"

class C():
    def __init__(self):
        print("__init__ of C called")
        self.c = "C"

class D(B, C):
    def __init__(self):
        print("__init__ of D called")
        super().__init__()

    def output(self):
        print(self.b, self.c)

d = D()
d.output()

I am getting the following error:

AttributeError: 'D' object has no attribute 'c'

Answer 1

super() will find the next method in the MRO sequence . This means that only one of the __init__ methods in your base classes is going to be called.

You can inspect the MRO (the Method Resolution Order ) by looking at the __mro__ attribute of a class:

>>> D.__mro__
(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class 'object'>)

so from D , the next class is B , followed by C and object . From D.__init__() , the super().__init__() expression will only call B.__init__() , and then because C.__init__() is never called , self.c is not set either.

You'll have to add more super() calls to your class implementations; it is safe to call object.__init__() with no arguments, so just use them everywhere here:

class B():
    def __init__(self):
        print("__init__ of B called")
        super().__init__()
        self.b = "B"

class C():
    def __init__(self):
        print("__init__ of C called")
        super().__init__()
        self.c = "C"

class D(B, C):
    def __init__(self):
        print("__init__ of D called")
        super().__init__()

    def output(self):
        print(self.b, self.c)

Now B.__init__ will invoke C.__init__ , and C.__init__ will call object.__init__ , and calling D().output() works:

>>> d = D()
__init__ of D called
__init__ of B called
__init__ of C called
>>> d.output()
B C