'''

Mimics ordinary dict objects, but allows cheap shallow copy operation

>>> d = CloneDict({1: 2})

>>> d

CloneDict({1: 2})

>>> d[42] = 42

>>> d == {1: 2, 42: 42}

True

>>> d2 = d.__copy__()

>>> del d[1]

>>> d

CloneDict({42: 42})

>>> d2 == {1: 2, 42: 42}

True

'''

__slots__ = [

'base', # persistent dict

'diff', # changes relative to it

]

def __init__(self, d=None):

if isinstance(d, CloneDict):

self.base = d.base = d.base.update(d.diff)

self.diff = {}

d.diff = {}

elif d is None:

self.base = PersistentDict()

self.diff = {}

else:

self.base = PersistentDict(d)

self.diff = {}

def __copy__(self):

return CloneDict(self)

def update(self, *args, **kwargs):

self.diff.update(*args, **kwargs)

def get(self, key, default=None):

if key not in self.diff:

return self.base.get(key, default)

value = self.diff[key]

if value is NO_VALUE:

return default

else:

return value

def __getitem__(self, key):

if key not in self.diff:

return self.base[key]

value = self.diff[key]

if value is NO_VALUE:

raise KeyError()

return value

def __setitem__(self, key, value):

self.diff[key] = value

def __delitem__(self, key):

self.diff[key] = NO_VALUE

def __contains__(self, key):

if key not in self.diff:

return key in self.base

return self.diff[key] is not NO_VALUE

def keys(self):

result = []

for k, v in self.diff.items():

if v is not NO_VALUE:

result.append(k)

for k in self.base.keys():

if k not in self.diff:

result.append(k)

return result

def items(self):

result = []

for kv in self.diff.items():

k, v = kv

if v is not NO_VALUE:

result.append(kv)

for kv in self.base.items():

k, v = kv

if k not in self.diff:

result.append(kv)

return result

def __len__(self):

result = 0

for k, v in self.diff.items():

if v is not NO_VALUE:

result += 1

for k in self.base.keys():

if k not in self.diff:

result += 1

return result

def __repr__(self):

return 'CloneDict({!r})'.format(dict(self.items()))

def __eq__(self, other):

if self is other:

return True

return set(self.items()) == set(other.items())

def __ne__(self, other):