ChainMap is a class from Python's collections module that groups multiple dictionaries into a single view. It allows to access and search data across several dictionaries without merging them into one.
Python
from collections import ChainMap
student = {"name": "Emma", "age": 21}
course = {"subject": "Python", "duration": "3 Months"}
data = ChainMap(student, course)
print(data)
OutputChainMap({'name': 'Emma', 'age': 21}, {'subject': 'Python', 'duration': '3 Months'})
Explanation:
- ChainMap(student, course) combines both dictionaries into a single view.
- The original dictionaries remain unchanged.
- Values from either dictionary can be accessed through the ChainMap object.
Common ChainMap Operations
ChainMap provides several useful methods for accessing and managing multiple dictionaries. These methods allow you to view keys and values, inspect underlying dictionaries and modify the dictionary lookup order.
1. keys(): returns all unique keys available across the dictionaries in the ChainMap.
Python
from collections import ChainMap
d1 = {"name": "Emma", "age": 21}
d2 = {"course": "Python", "city": "London"}
data = ChainMap(d1, d2)
print(list(data.keys()))
Output['course', 'city', 'name', 'age']
Explanation:
- keys() collects keys from all dictionaries in the ChainMap and duplicate keys appear only once.
- The result is returned as a view object, which is converted to a list here for display.
2. values(): returns the values associated with all keys in the ChainMap.
Python
from collections import ChainMap
d1 = {"name": "Emma", "age": 21}
d2 = {"course": "Python", "city": "London"}
data = ChainMap(d1, d2)
print(list(data.values()))
Output['Python', 'London', 'Emma', 21]
Explanation:
- values() returns the values corresponding to the keys in the ChainMap.
- Values are collected from all underlying dictionaries.
- If duplicate keys exist, the value from the first dictionary is used.
3. maps: displays all dictionaries contained in the ChainMap.
Python
from collections import ChainMap
d1 = {"name": "Emma"}
d2 = {"course": "Python"}
data = ChainMap(d1, d2)
print(data.maps)
Output[{'name': 'Emma'}, {'course': 'Python'}]
Explanation:
- maps returns a list containing all dictionaries inside the ChainMap.
- Dictionaries are displayed in the order they are searched.
- The first dictionary has the highest priority during key lookup.
4. new_child(): adds a new dictionary at the beginning of the ChainMap.
Python
from collections import ChainMap
d1 = {"name": "Emma"}
d2 = {"course": "Python"}
data = ChainMap(d1, d2)
res = data.new_child({"city": "London"})
print(res.maps)
Output[{'city': 'London'}, {'name': 'Emma'}, {'course': 'Python'}]
Explanation:
- new_child() creates a new ChainMap.
- The new dictionary is placed at the beginning.
- Keys in the new dictionary are searched before all existing dictionaries.
Accessing Values
One of the main purposes of ChainMap is to search for keys across multiple dictionaries as if they were a single dictionary.
Python
from collections import ChainMap
d1 = {"name": "Emma"}
d2 = {"course": "Python"}
data = ChainMap(d1, d2)
print(data["name"])
print(data["course"])
Explanation:
- data["name"] searches the first dictionary and returns "Emma".
- data["course"] is not found in the first dictionary, so ChainMap searches the next dictionary.
- This lookup continues until the key is found.
Handling Duplicate Keys
When the same key exists in multiple dictionaries, ChainMap always gives priority to the first dictionary.
Python
from collections import ChainMap
d1 = {"city": "London"}
d2 = {"city": "Paris"}
data = ChainMap(d1, d2)
print(data["city"])
Explanation:
- The key "city" exists in both dictionaries. ChainMap searches from left to right.
- Since "city" is found in the first dictionary, its value is returned.
- The value in the second dictionary is ignored during lookup.
Reversing Dictionary Lookup Order
We can reverse the order of dictionaries in a ChainMap by reversing the maps list.
Python
from collections import ChainMap
d1 = {"id": 101}
d2 = {"id": 202}
data = ChainMap(d1, d2)
print("Before:", data["id"])
data.maps = list(reversed(data.maps))
print("After:", data["id"])
