Python constructor does weird things with optional parameters

Question

Possible Duplicate:
least astonishment in python: the mutable default argument

Can you help me understand of the behaviour 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. Ok that was confusing... so look at an example I concocted below.

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 = NodeGood(1) # Root Node
  rootgood = ng
  ng.addchild(NodeGood(2)) # 1nd Child
  ng = ng.children[0]
  ng.addchild(NodeGood(3)) # 2nd Child

  print rootgood.value
  print rootgood.children[0].value
  print rootgood.children[0].children[0].value
  print rootgood.children[0].children

  print 'Bad Tree'
  nb = NodeBad(1) # Root Node
  rootbad = nb
  nb.addchild(NodeBad(2)) # 1st Child
  nb = nb.children[0]
  nb.addchild(NodeBad(3)) # 2nd Child

  print rootbad.value
  print rootbad.children[0].value
  print rootbad.children[0].children[0].value
  print rootbad.children[0].children

Answer 1

Mutable default arguments are a source of confusion.

See this answer: http://stackoverflow.com/questions/1132941/least-astonishment-in-python-the-mutable-default-argument

Answer 2

The problem is, the default value of an optional argument is only a single instance. So for example, if you say def __init__(self, value, c=[]):, that same list [] will be passed into the method each time an optional argument is used by calling code.

So basically you should only use immutable date types such as None for the default value of an optional argument. For example:

def __init__(self, value, c=None):

Then you could just create a new list in the method body:

if c == None:
  c = []