Python: inheritance of class data (if superclass object has been already initialized)

Question

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?

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?](http://stackoverflow.com/questions/222877/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

Martijn Pieters · Accepted Answer · 2013-10-24T19:29:30.093

2

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.

edited Oct 24 '13 at 19:29

answered Oct 24 '13 at 19:21

Martijn Pieters

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@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

Python: inheritance of class data (if superclass object has been already initialized)

1 Answers1