collections.ChainMap类合并字典或映射

## 使用update()方法或者ChainMap类合并字典或映射

 # 使用update()方法合并

 a = {'x': 1, 'z': 3}
 b = {'y': 2, 'z': 4}
 merged = dict(b) # 创建一个新字典
 print(merged)
 # {'y': 2, 'z': 4}
 merged.update(a) # 更新字典数据(合并)
 print(merged)
 # {'y': 2, 'z': 3, 'x': 1}

 a['x'] = 10     # 对原有字典的改变不会影响合并后的字典
 print(merged)
 # {'y': 2, 'z': 3, 'x': 1}

 # 使用collections.ChainMap()类
 from collections import ChainMap

 a = {'x': 1, 'z': 3}
 b = {'y': 2, 'z': 4}
 c = ChainMap(a, b)
 print(c)
 # ChainMap({'x': 1, 'z': 3}, {'y': 2, 'z': 4})

 # 数据的读取总是从第一个字典开始查找，找不到再从第二个中查找
 print(c['x'])
 #
 print(c['y'])
 #
 print(c['z'])
 #

 # 数据的增加或者删除操作总是针对第一个字典
 c['w'] = 4
 print(c)
 # ChainMap({'x': 1, 'z': 3, 'w': 4}, {'y': 2, 'z': 4})

 del c['w']
 print(c)
 # ChainMap({'x': 1, 'z': 3}, {'y': 2, 'z': 4})

 del c['y']      # 无法删除第二个字典中的键值对
 # KeyError: 'y'
 # KeyError: "Key not found in the first mapping: 'y'"

 # 对原有字典的操作会影响合并的字典
 a = {'x': 1, 'z': 3}
 b = {'y': 2, 'z': 4}
 c = ChainMap(a, b)
 print(c)
 # ChainMap({'x': 1, 'z': 3}, {'y': 2, 'z': 4})
 print(c['x'])
 #
 print(c['y'])
 #
 print(c['z'])
 #

 a['x'] = 5  # 修改原有字典的值
 print(c['x'])  # 合并的字典受到影响
 #

 # ChainMap()对象支持大多数字典的操作
 print(len(c))
 #
 print(list(c.keys()))
 # ['x', 'y', 'z']
 print(list(c.values()))
 # [5, 2, 3]

 # 添加或删除新的字典
 values = ChainMap()
 values['x'] = 1
 values = values.new_child()  # 添加一个空白字典
 print(values)
 # ChainMap({}, {'x': 1})
 values['x'] = 2  # 给空白字典添加内容
 print(values)
 # ChainMap({'x': 2}, {'x': 1})
 values = values.new_child()
 values['x'] = 3
 print(values)
 # ChainMap({'x': 3}, {'x': 2}, {'x': 1})

 # 删除第一个字典
 values = values.parents
 print(values)
 # ChainMap({'x': 2}, {'x': 1})
 values = values.parents
 print(values)
 # ChainMap({'x': 1})

参考资料：
　　Python Cookbook, 3rd edition, by David Beazley and Brian K. Jones (O’Reilly).