COW Shared Key Dicts

cow_dict.py

class CowDict(dict):
    _shared = {}

    def __init__(self):
        self._shared = CowDict._shared
        self._local = {}

    def __getitem__(self, key):
        value = self._local.get(key) or self._shared.get(key)
        if value:
            return value
        raise ValueError()

    def __repr__(self):
        return repr(
            dict(
                self.viewitems() | self._shared.viewitems()
                )
            )

    def __setitem__(self, key, value):
        if key in self._shared:
            self._local[key] = value
        else:
            self._shared[key] = value


foo1 = CowDict()
foo1['meh'] = 1
print foo1
foo2 = CowDict()
print foo2
assert foo1 == foo2