ch2.py

from collections import ChainMap
class DeepChainMap(ChainMap):
    def __setitem__(self, key, value):
        for mapping in self.maps:
            if key in mapping:
                mapping[key] = value
                return
        self.maps[0][key] = value

    def __delitem__(self, key):
        for mapping in self.maps:
            if key in mapping:
                del mapping[key]
                return
        raise KeyError(key)

d = DeepChainMap({'a': 1}, {'b': 2}, {'c': 3})
print (d)#Output:DeepChainMap({'a': 1}, {'b': 2}, {'c': 3})


#Updating existing key two levels down
d['c'] = 99
print (d)#Output:DeepChainMap({'a': 1}, {'b': 2}, {'c': 99})


#Adding new keys to first mapping
d['e'] = 5
print (d)#Output:DeepChainMap({'a': 1, 'e': 5}, {'b': 2}, {'c': 99})


#Deleting an existing key in second mapping(one level down)
del d['b']
print (d)#Output:DeepChainMap({'a': 1, 'e': 5}, {}, {'c': 99})