# 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
>

> 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})
>