Python - Extract values of Particular Key in Nested Values

Last Updated : 30 Jan, 2025

We are given a nested dictionary we need to extract the values of particular key. For example, we are having a dictionary d = {'a': 1,'b': {'a': 2, 'c': 3},'d': [{'a': 4}, {'a': 5}]} we need to to extract values from it. so that output becomes [1, 5, 4, 2].

Using a Stack

We can manually manage a stack to iterate through the nested dictionary and lists.

Python

d = {'a': 1,'b': {'a': 2, 'c': 3},'d': [{'a': 4}, {'a': 5}]}
key = 'a'
s = [d]
values = []

# Loop to process the stack until it's empty
while s:
    c = s.pop()
    if isinstance(c, dict):
        # Iterate through dictionary's key-value pairs
        for k, v in c.items():
            if k == key:
                values.append(v)
            elif isinstance(v, (dict, list)):
                s.append(v)
    # If the element is a list
    elif isinstance(c, list):
        
        s.extend(c)
print(values)

Output

[1, 5, 4, 2]

Explanation:

Code uses a stack to explore a nested dictionary/list, processing each element without recursion.
It collects values of specified key from dictionaries adding nested dictionaries/lists to stack for further exploration.

Using a Queue

Alternatively we can use a queue to explore structure in a breadth-first manner.

Python

from collections import deque  
d = {'a': 1,'b': {'a': 2, 'c': 3},'d': [{'a': 4}, {'a': 5}]}
key = 'a'
q = deque([d])
values = []

# Loop to process the queue until it's empty
while q:
    c = q.popleft()
    if isinstance(c, dict):
        # Iterate through the dictionary's key-value pairs
        for k, v in c.items():
            # If the key matches
            if k == key:
                values.append(v)
            # If the value is a dictionary or list 
            elif isinstance(v, (dict, list)):
                q.append(v)
    elif isinstance(c, list):
        q.extend(c)

print(values)

Output

[1, 2, 4, 5]

Explanation:

Code uses a queue (deque) to traverse a nested dictionary/list structure level by level processing each element iteratively.
It collects values of specified key from dictionaries and adds nested dictionaries/lists to queue for further exploration.

Iterate Through Structure

If we know structure is only a few levels deep (e.g., a dictionary containing lists or other dictionaries) we can use direct iteration without recursion or extra data structures.

Python

d = {'a': 1, 'b': {'a': 2, 'c': 3}, 'd': [{'a': 4}, {'a': 5}]}
key = 'a'
values = []

for k, v in d.items():
    if k == key:
        values.append(v)
    # If the value is a dictionary, iterate through its items
    elif isinstance(v, dict):
        for k2, v2 in v.items():
            # If the key matches in the nested dictionary append the value
            if k2 == key:
                values.append(v2)
    # If the value is a list iterate through its elements
    elif isinstance(v, list):
        for item in v:
            if isinstance(item, dict):
                for k2, v2 in item.items():
                    if k2 == key:
                        values.append(v2)

print(values)

Output

[1, 2, 4, 5]

Explanation:

Iterates through the top-level dictionary checking for the specified key and exploring nested dictionaries or lists.
Checks nested dictionaries and lists for key matches appending values to the result list.

Python - Extract values of Particular Key in Nested Values

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Python - Extract values of Particular Key in Nested Values

Using a Stack

Using a Queue

Iterate Through Structure

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