Inheritance in python 2.7

Question

I am attempting to build a framework for parsing a very specific text structure. The structure that I am processing is rich and has a known schema, similar to xml. I am attempting to build an framework to do the parsing. The text has various sections and I anticipate that more sections code be added in the future. To Compensate, I am attempting to build a series of derived classed that can be swapped in or out as needed.

I thought everything was going as planned, until I started coding up the first derived class. The base class has some functionality inside of __init__ that I was expecting that I would get for free in all of the concrete derived classes. That however, doesn't seem to be the case at all.

Here is a simple example to illustrate my problem: I would expect the output to be:

['memberA', 'memberB', 'memberC'], ['DerivedA', 'DerivedB', 'DerivedC']

class base(object):
    def __init__(self):
        members = [attr for attr in dir(self) if not callable(attr) and not attr.startswith("__")]
        print members

class test(base):
    def __init__(self):
        self.memberA = None
        self.memberB = None
        self.memberC = None


class test2(test):
    def __init__(self):
        self.DerivedA = None
        self.DerivedB = None
        self.DerivedC = None

t = test()
t2 = test2()

Can someone please explain to my, why the print functionality is not working as I expect it?

EDIT: in light of the answer below: I now have this question:

What if base.__init(self) instead looked like:

class base(object):
    def __init__(self, text):

would I have to define the derived class as:

class test(base):
    def __init__(self, text):
        base.__init__(self, text)

I was hoping to at least get the parameter object referance for free

unutbu · Accepted Answer · 2013-12-27 03:05:33Z

7

In Python, you must call the base class's __init__ explicitly inside test.__init__:

class test(base):
    def __init__(self):
        base.__init__(self)

Or, if you wish to support multiple inheritance, use super:

class test(base):
    def __init__(self):
        super(test, self).__init__()

If the base.__init__ looks like

class base(object):
    def __init__(self, text):

then test.__init__ should indeed look like

class test(base):
    def __init__(self, text):
        base.__init__(self, text)

See Guido van Rossum's blog for why self is explicit in Python.

PS. PEP8 recommends using CapWords for class names.

edited Dec 27, 2013 at 3:05

answered Dec 27, 2013 at 2:46

unutbu

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oyss · Accepted Answer · 2013-12-27 02:50:37Z

0

you are overwriting init in test2

following code will complete overwrite init in test. so there is no longer a print int the init function.

def __init__(self):
    self.DerivedA = None
    self.DerivedB = None
    self.DerivedC = None

answered Dec 27, 2013 at 2:50

oyss

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