Python – Convert Lists to Nested Dictionary

Last Updated : 05 Dec, 2023

Sometimes, while working with Python dictionaries, we can have a problem in which we need to convert lists to nestings, i.e. each list value represents a new nested level. This kind of problem can have applications in many domains including web development. Let’s discuss the certain way in which this task can be performed.

Convert Lists to Nested Dictionary in Python

Below are the ways by which we can convert lists to nested dictionary in Python:

Using zip() and list comprehension
Naive (Using for loop)
Using dictionary comprehension
Using the Itertools module

Convert Lists to Nested Dictionary Using zip() + list comprehension

The combination of the above functions can be combined to perform this task. In this, we iterate for a zipped list and render the nested dictionaries using list comprehension. Each element at corresponding positions in the three lists contributes to a key-value pair in the resulting dictionary, where the first list provides the outermost keys, the second list provides the intermediate keys, and the third list provides the values.

Python3

# initializing list
test_list1 = ["gfg", 'is', 'best']
test_list2 = ['ratings', 'price', 'score']
test_list3 = [5, 6, 7]
 
# printing original list
print("The original list 1 is : " + str(test_list1))
print("The original list 2 is : " + str(test_list2))
print("The original list 3 is : " + str(test_list3))
 
# Convert Lists to Nestings Dictionary
# Using list comprehension + zip()
res = [{a: {b: c}} for (a, b, c) in zip(test_list1, test_list2, test_list3)]
 
# printing result
print("The constructed dictionary : " + str(res))

Output

The original list 1 is : ['gfg', 'is', 'best']
The original list 2 is : ['ratings', 'price', 'score']
The original list 3 is : [5, 6, 7]
The constructed dictionary : [{'gfg': {'ratings': 5}}, {'is': {...

Time complexity: O(n)
Auxiliary space: O(n)

Convert Lists to Nested Dictionary in Python Using for loop

In this example, Python code utilizes a for loop to convert three separate lists (test_list1, test_list2, and test_list3) into a nested dictionary (res). Each element at corresponding positions in the three lists contributes to a key-value pair in the resulting dictionary, where the elements from test_list1 serve as outermost keys, elements from test_list2 serve as intermediate keys, and elements from test_list3 serve as values.

Python3

# initializing lists
test_list1 = ["gfg", 'is', 'best']
test_list2 = ['ratings', 'price', 'score']
test_list3 = [5, 6, 7]
 
# initializing dictionary
result_dict = {}
 
# printing original list
print("The original list 1 is : " + str(test_list1))
print("The original list 2 is : " + str(test_list2))
print("The original list 3 is : " + str(test_list3))
 
# iterating through the lists
for i in range(len(test_list1)):
    sub_dict = {test_list2[i]: test_list3[i]}
    result_dict[test_list1[i]] = sub_dict
 
# printing result
print("The constructed dictionary : " + str(result_dict))

Output

The original list 1 is : ['gfg', 'is', 'best']
The original list 2 is : ['ratings', 'price', 'score']
The original list 3 is : [5, 6, 7]
The constructed dictionary : {'gfg': {'ratings': 5}, 'is': {'pr...

Time Complexity: O(n), where n is the length of the lists
Auxiliary Space: O(n), where n is the length of the lists

Python Convert Lists to Nested Dictionary Using Dictionary Comprehension

In this example, the Python code utilizes dictionary comprehension and the zip() function to create a nested dictionary (res) from three lists (test_list1, test_list2, and test_list3). Each element at corresponding positions in the three lists contributes to a key-value pair in the resulting dictionary, where the elements from test_list1 serve as outermost keys, elements from test_list2 serve as intermediate keys, and elements from test_list3 serve as values.

Python3

# initializing lists
test_list1 = ["gfg", 'is', 'best']
test_list2 = ['ratings', 'price', 'score']
test_list3 = [5, 6, 7]
 
# using dictionary comprehension to create a nested dictionary
res = {a: {b: c} for (a, b, c) in zip(test_list1, test_list2, test_list3)}
 
# printing original list
print("The original list 1 is : " + str(test_list1))
print("The original list 2 is : " + str(test_list2))
print("The original list 3 is : " + str(test_list3))
 
# printing the resulting dictionary
print("The constructed dictionary: ", res)

Output

The original list 1 is : ['gfg', 'is', 'best']
The original list 2 is : ['ratings', 'price', 'score']
The original list 3 is : [5, 6, 7]
The constructed dictionary:  {'gfg': {'ratings': 5}, 'is': {'pr...

Time Complexity: O(n)
Auxiliary Space: O(n)

Convert Lists to Nested Dictionary in Python Using the itertools module

In this example, Python code utilizes the itertools.cycle() function to create an iterator that cycles through the provided lists (test_list1, test_list2, and test_list3). It then iterates over the combined elements of the lists using the zip() function, creating a nested dictionary (res) where elements from test_list1 serve as outermost keys, elements from test_list2 serve as intermediate keys, and elements from test_list3 serve as values.

Python3

import itertools
 
# Initializing lists
test_list1 = ["gfg", 'is', 'best']
test_list2 = ['ratings', 'price', 'score']
test_list3 = [5, 6, 7]
 
# Creating an iterator that cycles through the lists
cycle_lists = itertools.cycle([test_list1, test_list2, test_list3])
 
# Initializing an empty dictionary
res = {}
 
# printing original list
print("The original list 1 is : " + str(test_list1))
print("The original list 2 is : " + str(test_list2))
print("The original list 3 is : " + str(test_list3))
 
# Iterating over the combined elements of the lists
for a, b, c in zip(next(cycle_lists), next(cycle_lists), next(cycle_lists)):
    res[a] = {b: c}
 
# Printing the resulting dictionary
print("The constructed dictionary: ", res)

Output

The original list 1 is : ['gfg', 'is', 'best']
The original list 2 is : ['ratings', 'price', 'score']
The original list 3 is : [5, 6, 7]
The constructed dictionary:  {'gfg': {'ratings': 5}, 'is': {'pr...

Time Complexity: O(n)
Auxiliary Space: O(1)