Python constructor does weird things with optional parameters
- by christangrant
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
