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I was thinking of adding a recipes section to show how to extend or override the class:
class DjangoContext(ChainMap):
 def push(self):
 self.maps.insert(0, {})
 def pop(self):
 self.maps.pop(0)
class NestedScope(ChainMap):
 'Mutating methods that write to first matching dict'
 def __setitem__(self, key, value):
 '''Find the first matching *key* in chain and set its value.
 If not found, sets in maps[0].
 '''
 for m in self.maps:
 if key in m:
 break
 else:
 m = self.maps[0]
 try:
 cs = m.chain_set
 except AttributeError:
 m[key] = value
 else:
 cs(key, value)
 def __delitem__(self, key):
 '''Find and delete the first matching *key* in the chain.
 Raise KeyError if not found.
 '''
 for m in self.maps:
 if key in m:
 break
 try:
 cd = m.chain_del
 except AttributeError:
 del m[key]
 else:
 cd(key)
 def popitem(self):
 for m in self.maps:
 if m:
 break
 return m.popitem()
 def clear(self):
 for m in self.maps:
 m.clear()