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I faced with an inability of the inheritance of superclass attribute values. I have already called superclass constructor and now trying to check out the inherited values. 

class base:
    def __init__(self, x):
        self.x = x
        print(self.x)

class derive(base):
    def __init__(self):
        print(self.x + 1)


print("base class: ")
b = base(1)                           <-- Creating superclass instance 
print("derive class: ")
d = derived()                         <-- Inheriting. Failure.

Why can't I do this? Should I pass the underlying object to the inheriting object explicitly in order to get x attribute?

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2  
You need to call the base class __init__ from the derived class. There are many previous questions on this issue. –  BrenBarn Oct 24 '13 at 19:19
    
possible duplicate of How to use 'super' in Python? –  shx2 Oct 24 '13 at 19:20
    
@shx2: That question is rather Python 2-specific in its answers. –  Martijn Pieters Oct 24 '13 at 19:31

1 Answer 1

up vote 2 down vote accepted

b and d are not related; b is entirely a separate instance of the base class.

If you want to invoke the overridden initializer (__init__), then use the super() proxy object to access it:

class derive(base):
    def __init__(self):
        super().__init__(1)
        print(self.x + 1)

Note that you still need to pass in an argument to the initializer of the parent class. In the above example, I pass in a constant value 1 for the x parameter of the parent initializer.

Note that I used Python 3 specific syntax here; super() without arguments won't work in Python 2, where you also need to use object as a parent for the base class to make it a new-style class.

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1  
You should probably mention it won't work in Python 2. –  zero323 Oct 24 '13 at 19:27
    
@zero323: The OP is using print() functions; I stuck to Python 3 for that reason. –  Martijn Pieters Oct 24 '13 at 19:28
    
Sometimes I feel like the only one using print() and Python 2.x –  zero323 Oct 24 '13 at 19:46
    
@zero323: Don't, unless you use from __future__ import print_function. –  Martijn Pieters Oct 24 '13 at 19:48

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