Ways to create a dictionary of Lists – Python

Last Updated : 07 Jan, 2025

A dictionary of lists is a type of dictionary where each value is a list. These dictionaries are commonly used when we need to associate multiple values with a single key.

Initialize a Dictionary of Lists

This method involves manually defining a dictionary where each key is explicitly assigned a list of values.

Example:

Python

# Creating an empty dictionary d = {}  # Adding list as value d["1"] = [1, 2] d["2"] = ["Geeks", "For", "Geeks"]   print(d)

Output

{'1': [1, 2], '2': ['Geeks', 'For', 'Geeks']}

Explanation:

A dictionary where each key is paired with a list (e.g., [1, 2]).
This is the most straightforward way to define a dictionary of lists but requires hardcoding all the keys and values.

Let’s look at other methods of creating a dictionary of lists:

Table of Content

Using the zip() Function

zip() function can combine two lists (keys and lists of values) into a dictionary of lists. This method is efficient when the data is already structured as two separate lists.

Example:

Python

k = ["Fruits", "Vegetables", "Drinks"] val = [["Apple", "Banana"], ["Carrot", "Spinach"], ["Water", "Juice"]]  # Create a dictionary of lists using zip d = dict(zip(k, val))  print(d)

Output

{'Fruits': ['Apple', 'Banana'], 'Vegetables': ['Carrot', 'Spinach'], 'Drinks': ['Water', 'Juice']}

Explanation:

zip(keys, values) combines the keys and values lists into pairs (tuples).
dict() converts the resulting pairs into a dictionary.

Use defaultdict from collections

defaultdict automatically creates a default value for keys that don’t exist, making it ideal for building a dictionary of lists dynamically.

Example:

Python

from collections import defaultdict  # Initialize a defaultdict with list as the default type d = defaultdict(list)  # Add values to the dictionary d[1].append("Apple") d[2].append("Banana") d[3].append("Carrot")  print(d)

Output

defaultdict(<class 'list'>, {1: ['Apple'], 2: ['Banana'], 3: ['Carrot']})

Explanation:

defaultdict(list) automatically assigns an empty list as the default value for keys that don’t exist.
append() adds items to the lists associated with the keys. No need to check if the key exists beforehand.

Using setdefault()

setdefault() method simplifies handling missing keys by initializing a default list if the key doesn’t exist.

Example:

Python

li = [("Fruits", "Apple"), ("Fruits", "Banana"), ("Vegetables", "Carrot")]  # Initialize an empty dictionary d = {}  # Use setdefault to populate the dictionary for k, item in li:     d.setdefault(k, []).append(item)  print(d)

Output

{'Fruits': ['Apple', 'Banana'], 'Vegetables': ['Carrot']}

Explanation:

setdefault(category, []) checks if the key exists; if not, it creates the key with an empty list as its value.

Using Dictionary Comprehension

Dictionary comprehension is a concise way to create a dictionary of lists from structured data.

Example:

Python

li = [("Fruits", "Apple"), ("Fruits", "Banana"), ("Vegetables", "Carrot")]  # Create dictionary of lists using comprehension d = {k: [i for _, i in filter(lambda x: x[0] == k, li)] for k in set(k for k, _ in li)}  print(d)

Output

{'Fruits': ['Apple', 'Banana'], 'Vegetables': ['Carrot']}

Explanation:

filter(lambda x: x[0] == k, li) filters items matching the current key.
Comprehension iterates over unique keys and assigns a list of all matching items as the value.

Convert Lists to Nested Dictionary - Python

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Practice Tags :

Ways to create a dictionary of Lists – Python

Initialize a Dictionary of Lists

Using the zip() Function

Use defaultdict from collections

Using setdefault()

Using Dictionary Comprehension

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