code: def update_nested_dict(a, b):
      
      for k, v in b.iteritems():
          if isinstance(v, dict):
              d = a.setdefault(k, {})
              update_nested_dict(d, v)
          else:
              a[k] = v
      return a
docs: update nested dict `a` with another dict b.
      usage::
  
          >>> a = {'x' : { 'y': 1}}
          >>> b = {'x' : {'z':2, 'y':3}, 'w': 4}
          >>> update_nested_dict(a,b)
          {'x': {'y': 3, 'z': 2}, 'w': 4}