Trouble with Python Inheritance

Question

The code that I'm acttualy having the problem with is very long, so I made an example that displays my problem.

I have two classes that inherit from a base-class (BaseClass). Both of these classes add some elements to self.Dict. However, they seem to cross contaminate elements. I was expecting c0.Dict to return {'class0': 0} and c1.Dict to return {'class1': 1}. However they both return {'class0': 0, 'class1': 1}. Why do they cross contaminate?

class BaseClass :
    def __init__ (self, _dict={}) :
        self.Dict = _dict

class Class0 (BaseClass) :
    def __init__ (self) :
        BaseClass.__init__(self)
        self.Dict['class0'] = 0

class Class1 (BaseClass) :
    def __init__ (self) :
        BaseClass.__init__(self)
        self.Dict['class1'] = 1

c0 = Class0()
c1 = Class1()

print c0.Dict
print c1.Dict

bobflux · Accepted Answer · 2011-05-14 23:03:36Z

9

You hit a python gotcha : mutable default arguments.

http://blog.objectmentor.com/articles/2008/05/22/pythons-mutable-default-problem

class BaseClass :
    def __init__ (self, _dict=None) :
        self.Dict = _dict or {}

answered May 14, 2011 at 23:03

bobflux

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3 Comments

rectangletangle Over a year ago

LOL, I was not expecting that. I was practically pulling my hair out trying to figure out where some strange side effects were coming from in my program. Thanks!

rectangletangle Over a year ago

Also, the fix you posted fixes the problem. Thanks for the quick reply too.

bobflux Over a year ago

yeah, I felt like you did when I hit that one, ie WTF ? (a few hours later) damn, ok...

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