Python - Append to list nested in dict subclass

Question

I have a dict subclass whose job is to dynamically add nested dict key if it not exists and do list append if append is called:

class PowerDict(dict):
    def __getitem__(self, item):
        try:
            return dict.__getitem__(self, item)
        except KeyError:
            value = self[item] = type(self)()
            return value
    def append(self,item):
        if type(self) != list:
            self = list()
            self.append(item)

so

a = PowerDict()
a['1']['2'] = 3

produce output:

a = {'1': {'2': 3}}

However, sometime i need to do something like this:

b = PowerDict()
b['1']['2'].append(3)
b['1']['2'].append(4)

should produce output:

b = {'1': {'2': [3, 4]}}

but above code produce output:

{'1': {'2': {}}}

What i am missing?

Answer 1

class PowerDict(dict):
    # http://stackoverflow.com/a/3405143/190597 (gnibbler)
    def __init__(self, parent = None, key = None):
        self.parent = parent
        self.key = key
    def __missing__(self, key):
        self[key] = PowerDict(self, key)
        return self[key]
    def append(self, item):
        self.parent[self.key] = [item]
    def __setitem__(self, key, val):
        dict.__setitem__(self, key, val)
        try:
            val.parent = self
            val.key = key
        except AttributeError:
            pass

a = PowerDict()
a['1']['2'] = 3
print(a)

b = PowerDict()
b['1']['2'].append(3)
b['1']['2'].append(4)
print(b)

a['1']['2'] = b
a['1']['2'].append(5)
print(a['1']['2'])

yields

{'1': {'2': 3}}
{'1': {'2': [3, 4]}}
[5]