Why is instance variable behaving like a class variable in Python? [duplicate]

Answer 1

As indicated in Martijn's comment and kindall's answer, you are running into the mutable default argument behavior which bites most Python developers at some point, here is how you can modify Node.__init__() so that it works the way you expect:

class Node(object):
    def __init__(self, value = 0, children = None):
        self.val = value
        if children is None:
            self.children = {}
        else:
            self.children = children

Answer 2

When you define the function __init__ you give it a dictionary as a default argument. That dictionary is created once (when you define the function) and then used every time __init__ is called.

More information: http://effbot.org/zone/default-values.htm

Answer 3

You're assigning the same dictionary to children on every instance.