Why does initializing a variable via a python default variable keep state across object instantiation?

I hit an interesting python bug today in which instantiating a class repeatedly appears to be holding state. In later instantiation calls the variable is already defined.

I boiled down the issue into the following class/shell interaction. I realize that this is not the best way to initialize a class variable, but it sure should not be behaving like this. Is this a true bug or is this a "feature"? :D

tester.py:

class Tester():
        def __init__(self):
                self.mydict = self.test()

        def test(self,out={}):
                key = "key"
                for i in ['a','b','c','d']:
                        if key in out:
                                out[key] += ','+i
                        else:   
                                out[key] = i 
                return out

Python prompt:

Python 2.6.6 (r266:84292, Oct  6 2010, 00:44:09) 
[GCC 4.2.1 (Apple Inc. build 5664)] on darwin
>>> import tester
>>> t = tester.Tester()
>>> print t.mydict
{'key': 'a,b,c,d'}
>>> t2 = tester.Tester()
>>> print t2.mydict
{'key': 'a,b,c,d,a,b,c,d'}