def union_dict(*objs): 
  _keys = set(sum([obj.keys() for obj in objs],[])) 
  _total = {} 
  for _key in _keys: 
    _total[_key] = sum([obj.get(_key,0) for obj in objs]) 
  return _total 
 
obj1 = {'a':1,'b':2,'c':3} 
obj2 = {'a':1,'b':3,'d':4} 
print union_dict(obj1,obj2) 

{'a': 2, 'c': 3, 'b': 5, 'd': 4}

>>sum([[1,2,3],[4,5]],[])
[1,2,3,4,5]

def symmetric_difference(_oldobj,_newobj): 
  _oldkeys = _oldobj.keys() 
  _newkeys = _newobj.keys() 
  _diff = {} 
  for _key in set(_oldkeys + _newkeys): 
    _val = _newobj.get(_key,0) - _oldobj.get(_key,0) 
    if _val: 
      _diff[_key] = _val  
  return _diff  
 
oldobj = {'a':1,'b':2,'c':3} 
newobj = {'a':1,'b':3,'d':4} 
print symmetric_difference(oldobj,newobj) 

{'b': 1, 'd': 4,'c': -3}