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.



In [2]:

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



In [3]:

    
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)

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.



In [6]:

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

del c['x']

print(a)
print(b)









    



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



In [7]:

    
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.



In [8]:

    
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'])

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.



In [9]:

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









    Out[9]:





1

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



In [10]:

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

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



In [11]:

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