Extract Dictionary Values as a Python List

To extract dictionary values from a list, we iterate through each dictionary, check for the key's presence, and collect its value. The result is a list of values corresponding to the specified key across all dictionaries.
For example, given data = {'a': 1, 'b': 2, 'c': 3}, the output will be [1, 2, 3]. In this article, we will explore different methods to achieve this.

Using list comprehension

List comprehension is a efficient way to extract values from dictionaries in a list. By checking if the key exists, it safely retrieves and adds the corresponding value to the result, ensuring efficient and concise data processing.

li = [
    {'name': 'improvement', 'letters': 25, 'city': 'Hyderabad'},
    {'name': 'geeksforgeeks', 'letters': 30, 'city': 'United states'},
    {'name': 'author', 'letters': 22, 'city': 'Japan'}
]

k = 'name' # key

# Extract values
ans = [i[k] for i in li if k in i]
print(ans)

['improvement', 'geeksforgeeks', 'author']

Explanation:

• for i in li : This iterates through each dictionary in the list li .
• if k in i: This ensures the key k exists in the current dictionary to avoid errors.
• i[k]: This retrieves the value for the key k .

Using map()

By combining map with filter, we can efficiently extract specific values from a list of dictionaries without using explicit loops. This approach is particularly useful when working with large datasets as it enhances performance.

li = [
    {'name': 'improvement', 'letters': 25, 'city': 'Hyderabad'},
    {'name': 'geeksforgeeks', 'letters': 30, 'city': 'United states'},
    {'name': 'author', 'letters': 22, 'city': 'Japan'}
]

k = 'city' # key

ans = list(map(lambda i: i[k], filter(lambda i: k in i , li)))
print(ans)

['Hyderabad', 'United states', 'Japan']

Explanation:

• filter ()checks if the keyk exists in each dictionary and returns only dictionaries containing the key k .
• map () extracts the value for the keyk from each dictionary returned by filter.

Using generator expression

Generator expressions provide an efficient way to extract values from dictionaries in a list, as they only generate values when needed. This is ideal for large datasets, as it minimizes memory usage by avoiding the creation of an entire intermediate list in memory.

li = [
    {'name': 'improvement', 'letters': 25, 'city': 'Hyderabad'},
    {'name': 'geeksforgeeks', 'letters': 30, 'city': 'United states'},
    {'name': 'author', 'letters': 22, 'city': 'Japan'}
]

k = 'name' # key

ans = list(i[k] for i in li if k in i)
print(ans)

['improvement', 'geeksforgeeks', 'author']

Explanation:

• Generator expression: This loops through each dictionary in li and checks if the key k exists.
• If condition: if k in iensures the key is present in the dictionary.
• i[k] retrieves the value associated with the key k .
• list() converts the values generated by the generator expression into a list.

Using reduce()

reduce from functools lets us apply a custom function to combine values in an iterable. It's useful for advanced cases where we need custom logic though, not as common as list comprehension for simpler tasks.

from functools import reduce

li = [
    {'name': 'improvement', 'letters': 25, 'city': 'Hyderabad'},
    {'name': 'geeksforgeeks', 'letters': 30, 'city': 'United states'},
    {'name': 'author', 'letters': 22, 'city': 'Japan'}
]

k = 'name'  # key

ans = reduce(lambda acc, i: acc + [i[k]] if k in i else acc, li, [])
print(ans)

['improvement', 'geeksforgeeks', 'author']

Explanation:

• reduce applies the lambda function across all elements of the list li .
• Lambda adds the value of i[k] toaccif k exists in the dictionary, otherwise keeps acc unchanged.