Update a Dictionary with the Values from a Dictionary List

Last Updated : 15 Jul, 2025

The task of updating a dictionary with values from a list of dictionaries involves merging multiple dictionaries into one where the keys from each dictionary are combined and the values are updated accordingly. For example, we are given an dictionary d = {"Gfg": 2, "is": 1, "Best": 3} and a list of dictionaries li = [{'for': 3, 'all': 7}, {'geeks': 10}, {'and': 1, 'CS': 9}], the goal is to merge all the dictionaries into a single one. After the merge, the updated dictionary will contain all key-value pairs from d and li, resulting in a final dictionary that looks like {'Gfg': 2, 'is': 1, 'Best': 3, 'for': 3, 'all': 7, 'geeks': 10, 'and': 1, 'CS': 9}.

Using chainMap

ChainMap from the collections module that allows us to chain multiple dictionaries together, creating a single view of the data. This method provides an efficient way to merge a dictionary with a list of dictionaries without the need for copying or creating a new dictionary. It efficiently links the dictionaries and provides a unified view of their keys and values.

Python 
from collections import ChainMap
d = {"Gfg": 2, "is": 1, "Best": 3} 

a = [{'for': 3, 'all': 7}, {'geeks': 10}, {'and': 1, 'CS': 9}] 

res = {**d, **dict(ChainMap(*a))}
print(str(res))

Output
{'Gfg': 2, 'is': 1, 'Best': 3, 'and': 1, 'CS': 9, 'geeks': 10, 'for': 3, 'all': 7}

Explanation: ChainMap(*a) merges the dictionaries in a, and dict(ChainMap(*a)) converts the combined view into a regular dictionary. The original dictionary d is merged with this result using the unpacking syntax {**d, **dict(ChainMap(*a))}.

Using dict.update()

dict.update() updates the dictionary with the key-value pairs from another dictionary. It modifies the original dictionary in place, which is highly efficient as it avoids unnecessary object creation. By iterating over the list of dictionaries we can update the original dictionary with each one.

Python 
d = {"Gfg": 2, "is": 1, "Best": 3}
a = [{'for': 3, 'all': 7}, {'geeks': 10}, {'and': 1, 'CS': 9}]

for i in a: # Looping through each dictionary in the list 'a'
    d.update(i)

print(str(d))

Output
{'Gfg': 2, 'is': 1, 'Best': 3, 'for': 3, 'all': 7, 'geeks': 10, 'and': 1, 'CS': 9}

Explanation: This code iterates over each dictionary in the list "a" using update() to add or update key-value pairs in d. After merging all dictionaries, the final updated dictionary d is printed.

Using dictionary comprehension

Dictionary comprehension is a concise way to create new dictionaries. It allows us to merge multiple dictionaries by iterating over a list of dictionaries and combining their key-value pairs into a single dictionary. This method can also be used to conditionally select or modify keys and values during the merging process.

Python 
d = {"Gfg": 2, "is": 1, "Best": 3}
a = [{'for': 3, 'all': 7}, {'geeks': 10}, {'and': 1, 'CS': 9}]

res = {**d, **{k: v for dic in a for k, v in dic.items()}}
print(str(res))

Output
{'Gfg': 2, 'is': 1, 'Best': 3, 'for': 3, 'all': 7, 'geeks': 10, 'and': 1, 'CS': 9}

Explanation: dictionary comprehension iterates through each dictionary in "a" and extracts all key-value pairs using dic.items(), combining them into a single dictionary. The merged dictionary is then combined with d using the unpacking syntax {**d, **{k: v for dic in a for k, v in dic.items()}},

Using reduce()

reduce() from the functools module applies a binary function, like merging dictionaries, cumulatively to the items of an iterable. This method allows us to merge all dictionaries in the list by combining them into one result. It is a functional programming approach compared to the others.

Python 
from functools import reduce

d = {"Gfg": 2, "is": 1, "Best": 3}
a = [{'for': 3, 'all': 7}, {'geeks': 10}, {'and': 1, 'CS': 9}]

res = reduce(lambda x, y: {**x, **y}, [d] + a)
print(str(res))

Output
{'Gfg': 2, 'is': 1, 'Best': 3, 'for': 3, 'all': 7, 'geeks': 10, 'and': 1, 'CS': 9}

Explanation: reduce() with a lambda function {**x, **y} to iteratively merge d with each dictionary in a, resulting in a single merged dictionary. This dictionary containing all key-value pairs from d and a is then printed.

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