Question

code:

class Node:
    def __init__(self, key, children=[]):
        self.key = key
        self.children = children

    def __repr__(self):
        return self.key

execute:

root = Node("root")
child = Node("child")
root.children.append(child)
print child.children
print root.children[0].children

result:

[child]
[child]

This is really weird, why?

Python’s version is 2.7.2.

Answer 1

You shouldn’t use mutable objects as default value of arguments (unless you exactly know what you’re doing). See this article for explanation.

Instead use:

class Node:
    def __init__(self, key, children=None):
        self.key = key
        self.children = children if children is not None or []