Python multi-inheritance, __init__

Question

Regarding multiple parent inheritance, when I call the super.__init__, why doesn't parent2's __init__ function get called? Thanks.

class parent(object):     var1=1     var2=2     def __init__(self,x=1,y=2):         self.var1=x         self.var2=y  class parent2(object):     var4=11     var5=12     def __init__(self,x=3,y=4):         self.var4=x         self.var5=y      def parprint(self):         print self.var4         print self.var5  class child(parent, parent2):     var3=5     def __init__(self,x,y):         super(child, self).__init__(x,y)  childobject = child(9,10) print childobject.var1 print childobject.var2 print childobject.var3 childobject.parprint() 

Output is

9 10 5 11 12 

Answer 1

If you want to use super in child to call parent.__init__ and parent2._init__, then both parent __init__s must also call super:

class parent(Base):     def __init__(self,x=1,y=2):         super(parent,self).__init__(x,y)     class parent2(Base):     def __init__(self,x=3,y=4):         super(parent2,self).__init__(x,y) 

See "Python super method and calling alternatives" for more details on the sequence of calls to __init__ caused by using super.

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class Base(object):      def __init__(self,*args):         pass  class parent(Base):     var1=1     var2=2     def __init__(self,x=1,y=2):         super(parent,self).__init__(x,y)                 self.var1=x         self.var2=y  class parent2(Base):     var4=11     var5=12     def __init__(self,x=3,y=4):         super(parent2,self).__init__(x,y)         self.var4=x         self.var5=y      def parprint(self):         print self.var4         print self.var5  class child(parent, parent2):     var3=5     def __init__(self,x,y):         super(child, self).__init__(x,y)   childobject = child(9,10) print childobject.var1 print childobject.var2 print childobject.var3 childobject.parprint() 

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You might be wondering, "Why use Base?". If parent and parent2 had inherited directly from object, then super(parent2,self).__init__(x,y) would call object.__init__(x,y). That raises a TypeError since object.__init__() takes no parameters.

To workaround this issue, you can make a class Base which accepts arguments to __init__ but does not pass them on to object.__init__. With parent and parent2 inheriting from Base, you avoid the TypeError.

Answer 2

Because parent is next in method resolution order (MRO), and it never uses super() to call into parent2.