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Why do class methods share mutable defaults between instances?

Ron Teeter ron.teeter at supplysolution.com
Tue Apr 30 18:11:37 EDT 2002 
Greetings,

Today I made a rather perplexing discovery.  It appears that mutable defaults 
in class methods are shared between instances.  Here is a simple example of 
the scenario.

class Demo:
    def __init__(self,name,dict={},list=[]):
        self.name = name
        self.dict = dict
        self.list = list
        self.dict[name] = name
        self.list.append(name)

def dump(name,dict={},list=[]):
    print name,dict,list

for name in ['A1','B1','C1']:
    tmp = Demo(name)
    dump(tmp.name,tmp.dict,tmp.list)

>>>
A1 {'A1': 'A1'} ['A1']
B1 {'B1': 'B1', 'A1': 'A1'} ['A1', 'B1']
C1 {'C1': 'C1', 'B1': 'B1', 'A1': 'A1'} ['A1', 'B1', 'C1']
>>>

My intent was that each time Demo was instanciated a new dictionary would be 
assigned to 'dict' if there was no corresponding argument.  However, the 
behavior is as though the {} were replaced with a class constant.

In order to avoid this behavior I had to reorganize Demo as follows:

class Demo2:
    def __init__(self,name,dict=None,list=None):
        if dict==None:
            dict = {}
        if list==None:
            list = []
        self.name = name
        self.dict = dict
        self.list = list
        self.dict[name] = name
        self.list.append(name)

I am wondering if this is a bug or a subtle usage error on my part.  I would 
expect that syntax on functions would be valid for class methods as well, but 
in this case you get drastically different behaviors.

Thanks,

Ron Teeter
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