Adding Custom Values Key in a List of Dictionaries in Python

In this article, we will learn how to add a custom key with corresponding values to a list of dictionaries in Python. This is a common task when working with structured data.

Dictionaries in Python are collections of unordered items stored in the form of key-value pairs inside curly braces ?

dictionary = {"key": "value", "name": "John"}
print(dictionary)

{'key': 'value', 'name': 'John'}

Suppose we have a list of dictionaries called dict_list and a key new_key, with its values provided in a separate list value_list. The goal is to add new_key to each dictionary in dict_list, assigning it the corresponding value from value_list.

Example Scenario

# Initial list of dictionaries
dict_list = [
    {'name': 'John', 'location': 'Hyderabad'},
    {'name': 'Jane', 'location': 'Bangalore'}
]

# Key and values to add
new_key = 'age'
value_list = [22, 23]

print("Original:", dict_list)
print("Adding key:", new_key)
print("Values to add:", value_list)

Original: [{'name': 'John', 'location': 'Hyderabad'}, {'name': 'Jane', 'location': 'Bangalore'}]
Adding key: age
Values to add: [22, 23]

We can add custom key-value pairs to each dictionary using several approaches ?

Using enumerate() Function with For Loop

The enumerate() function adds a counter to each item of an iterable. We use it to access both the index and dictionary simultaneously ?

# Initializing data
dict_list = [
    {'name': 'John', 'location': 'Hyderabad'},
    {'name': 'Jane', 'location': 'Bangalore'}
]

new_key = 'age'
value_list = [22, 23]

# Adding new key-value pair using enumerate
for i, d in enumerate(dict_list):
    d[new_key] = value_list[i]
    
print(dict_list)

[{'name': 'John', 'location': 'Hyderabad', 'age': 22}, {'name': 'Jane', 'location': 'Bangalore', 'age': 23}]

Using List Comprehension with enumerate()

List comprehension provides a concise way to create new lists. We use dictionary unpacking (**dic) to preserve existing keys ?

# Initializing data
dict_list = [
    {'name': 'John', 'location': 'Hyderabad'},
    {'name': 'Jane', 'location': 'Bangalore'}
]

new_key = 'age'
value_list = [22, 23]

# Creating new list with added key-value pairs
dict_list = [{**dic, new_key: value_list[idx]} for idx, dic in enumerate(dict_list)]
    
print(dict_list)

[{'name': 'John', 'location': 'Hyderabad', 'age': 22}, {'name': 'Jane', 'location': 'Bangalore', 'age': 23}]

Using List Comprehension with zip()

The zip() function pairs elements from multiple iterables by index position. This allows us to combine dictionaries and values directly ?

# Initializing data
dict_list = [
    {'name': 'John', 'location': 'Hyderabad'},
    {'name': 'Jane', 'location': 'Bangalore'}
]

new_key = 'age'
value_list = [22, 23]

# Adding new key-value pair using zip()
dict_list = [{**dic, new_key: val} for dic, val in zip(dict_list, value_list)]

print(dict_list)

[{'name': 'John', 'location': 'Hyderabad', 'age': 22}, {'name': 'Jane', 'location': 'Bangalore', 'age': 23}]

Using the map() Function

The map() function applies a function to each item in an iterable. We use a lambda function to add the new key-value pair ?

# Initializing data
dict_list = [
    {'name': 'John', 'location': 'Hyderabad'},
    {'name': 'Jane', 'location': 'Bangalore'}
]

new_key = 'age'
value_list = [22, 23]

# Using map() with lambda to add key-value pairs
dict_list = list(map(
    lambda dic, val: {**dic, new_key: val}, 
    dict_list, 
    value_list
))

print(dict_list)

[{'name': 'John', 'location': 'Hyderabad', 'age': 22}, {'name': 'Jane', 'location': 'Bangalore', 'age': 23}]

Comparison

Conclusion

Use zip() with list comprehension for the cleanest syntax when creating new dictionaries. Use enumerate() with a for loop when modifying dictionaries in-place. The map() function offers a functional programming approach for complex transformations.