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Possible Duplicate: least astonishment in python: the mutable default argument I want to understand of the behavior and implications of the python __init__ constructor. It seems like when there is an optional parameter and you try and set an existing object to a new object the optional value of the existing object is preserved and copied. Look at an example: In the code below I am trying to make a tree structure with nodes and possibly many children . In the first class NodeBad , the constructor has two parameters, the value and any possible children. The second class NodeGood only takes the value of the node as a parameter. Both have an addchild method to add a child to a node. When creating a tree with the NodeGood class, it works as expected. However, when doing the same thing with the NodeBad class, it seems as though a child can only be added once! The code below will result in the following output: Good Tree 1 2 3 [< 3 >] Bad Tree 1 2 2 [< 2 >, < 3 >] Que Pasa? Here is the Example: #!/usr/bin/python class NodeBad: def __init__(self, value, c=[]): self.value = value self.children = c def addchild(self, node): self.children.append(node) def __str__(self): return '< %s >' % self.value def __repr__(self): return '< %s >' % self.value class NodeGood: def __init__(self, value): self.value = value self.children = [] def addchild(self, node): self.children.append(node) def __str__(self): return '< %s >' % self.value def __repr__(self): return '< %s >' % self.value if __name__ == '__main__': print 'Good Tree' ng = No

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