Python: Default parameter vs. local value difference

Question

Here is some code (Python 2.7):

class Test:
    @staticmethod
    def BuildSample():
        rootItem = Test('root')
        childA = Test('ChildA')
        rootItem.AppendChildren([childA])
        childA.AppendChildren([Test('Child' + str(i)) for i in range(3)])
        return rootItem     

    def __init__(self, name):
        self.name = name
        self.children = []

    def AppendChildren(self, items):
        self.children.extend(items)

    def GetChildren(self):
        return self.children

    def GetName(self):
        return self.name

sample = Test.BuildSample()
print 'sample.GetName() = ' + sample.GetName()
print 'sample.GetChildren() has ' + str(len(sample.GetChildren())) + ' children.'
print 'childA.GetName() = ' + sample.GetChildren()[0].GetName()
print 'childA.GetChildren() has ' + str(len(sample.GetChildren()[0].GetChildren())) + ' children.'

Here is the output:

sample.GetName() = root
sample.GetChildren() has 1 children.
childA.GetName() = ChildA
childA.GetChildren() has 3 children.

This is what I would expect. Now, if I change the code above by replacing the constructor with this version:

def __init__(self, name, children=[]):
    self.name = name
    self.children = children

... I get the following output:

sample.GetName() = root
sample.GetChildren() has 4 children.
childA.GetName() = ChildA
childA.GetChildren() has 4 children.

What's happening is that the Child0, Child1, and Child2 items are being added to the children list of BOTH the root item AND the childA item.

My question is, why? What is the semantic difference between these two methods when the optional children parameter isn't given?

Python: Default parameter vs. local value difference

0 Answers0