Combining Multiple Mappings into a Single Mapping

Problem

• You have multiple dictionaries or mappings that you want to logically combine into a single mapping to perform certain operations, such as looking up values or checking for the existence of keys.

Solution

• An easy way to do this is to use the ChainMap class from the collections module.

a = {'x': 1, 'z': 3 }
b = {'y': 2, 'z': 4 }

from collections import ChainMap

c = ChainMap(a,b)
print(c['x']) # Outputs 1 (from a) 
print(c['y']) # Outputs 2 (from b) 
print(c['z']) # Outputs 3 (from a)

1
2
3

Discussion

• A ChainMap takes multiple mappings and makes them logically appear as one. However, the mappings are not literally merged together.
• If there are duplicate keys, the values from the first mapping get used. Thus, the entry c['z'] in the example would always refer to the value in dictionary a, not the value in dictionary b.
• Operations that mutate the mapping always affect the first mapping listed.

c['z'] = 10
c['w'] = 40

del c['x']

print(a)
print(b)

{'z': 10, 'w': 40}
{'y': 2, 'z': 4}

del c['y']

---------------------------------------------------------------------------

KeyError                                  Traceback (most recent call last)

/Users/tvu/anaconda/envs/py35/lib/python3.5/collections/__init__.py in __delitem__(self, key)
    928         try:
--> 929             del self.maps[0][key]
    930         except KeyError:


KeyError: 'y'


During handling of the above exception, another exception occurred:


KeyError                                  Traceback (most recent call last)

<ipython-input-7-df3e26fa6544> in <module>()
----> 1 del c['y']


/Users/tvu/anaconda/envs/py35/lib/python3.5/collections/__init__.py in __delitem__(self, key)
    929             del self.maps[0][key]
    930         except KeyError:
--> 931             raise KeyError('Key not found in the first mapping: {!r}'.format(key))
    932 
    933     def popitem(self):


KeyError: "Key not found in the first mapping: 'y'"

• As an alternative to ChainMap, you might consider merging dictionaries together using the update() method.

a = {'x': 1, 'z': 3 }
b = {'y': 2, 'z': 4 }
merged = dict(b)
merged.update(a)

print(merged['x'])
print(merged['y'])
print(merged['z'])

1
2
3

• This works, but it requires you to make a completely separate dictionary object (or destructively alter one of the existing dictionaries). Also, if any of the original diction‐ aries mutate, the changes don’t get reflected in the merged dictionary.

a['x'] = 13
merged['x']

1

• A ChainMap uses the original dictionaries, so it doesn’t have this behavior.

a = {'x': 1, 'z': 3 }
b = {'y': 2, 'z': 4 }

merged = ChainMap(a, b)
print(merged['x'])

1

a['x'] = 42
print(merged['x']) # Notice change to merged dicts
print(a['x'])

42
42