问题：在逻辑上将多个字典或映射合并为一个单独的映射结构，以此执行某些特定的操作，比如查找值或者检查键是否存在
解决方案：利用collections模块中的ChainMap类
ChainMap可接受多个映射然后在逻辑上使它们表现为一个单独的映射结构。这些映射在字面上并不会合并在一起。相反，ChainMap只是简单地维护一个记录底层映射关系的列表，然后重定义常见的字典操作来扫描这个列表。
# example.py
#
# Example of combining dicts into a chainmap a = {'x': 1, 'z': 3 }
b = {'y': 2, 'z': 4 } # (a) Simple example of combining
from collections import ChainMap
c = ChainMap(a,b) #如果有重复的键，那么会采用第一个映射中所对应的值。
print(c['x']) # Outputs 1 (from a)
print(c['y']) # Outputs 2 (from b)
print(c['z']) # Outputs 3 (from a) # Output some common values
print('len(c):', len(c))
print('c.keys():', list(c.keys()))
print('c.values():', list(c.values())) # Modify some values
c['z'] = 10
c['w'] = 40 print("a:", a)
del c['x']
print("a:", a) # Example of stacking mappings (like scopes)
values = ChainMap()
values['x'] = 1 # Add a new mapping
values = values.new_child()
values['x'] = 2 # Add a new mapping
values = values.new_child()
values['x'] = 3 print(values)
print(values['x']) # Discard last mapping
values = values.parents
print(values)
print(values['x']) # Discard last mapping
values = values.parents
print(values)
print(values['x'])
>>> ================================ RESTART ================================
>>>
1
2
3
len(c): 3
c.keys(): ['y', 'x', 'z']
c.values(): [2, 1, 3]
a: {'x': 1, 'z': 10, 'w': 40}
a: {'z': 10, 'w': 40}
ChainMap({'x': 3}, {'x': 2}, {'x': 1})
3
ChainMap({'x': 2}, {'x': 1})
2
ChainMap({'x': 1})
1
>>>
另外ChainMap操作的是原始字典，可以避免一些令人不悦的行为，例如其中任何一个原始字典的修改无法反应到合并后的字典中。
>>> a={'x':1,'z':3}
>>> b={'y':2,'z':4}
>>> merged=ChainMap(a,b)
>>> merged
ChainMap({'x': 1, 'z': 3}, {'y': 2, 'z': 4})
>>> merged['x']
1
>>> a['x']=55
>>> merged['x']
55
>>> merged
ChainMap({'x': 55, 'z': 3}, {'y': 2, 'z': 4})
>>>