import collections

class MySimpleMapping(collections.MutableMapping):
    def __init__(self, *args, **kw):
        self.inner = dict(*args, **kw)
    def __contains__(self, item):
        return item in self.inner
    def __iter__(self):
        return iter(self.inner)
    def __len__(self):
        return len(self.inner)
    def __getitem__(self, index):
        return self.inner[index]
    def __setitem__(self, key, value):
        self.inner[key] = value
    def __delitem__(self, key):
        del self.inner[key]

x = MySimpleMapping(red=5)
y = x.keys()
z = x.keys() | {'orange'}
x['blue'] = 7
print(list(z))
print(list(z))