Ways to create a dictionary of lists in python

A dictionary in Python is a collection of data stored in the form of key-value pairs. We can assign different datatypes as the value for a key. Lists store data in sequences that can be traversed and manipulated. When combining these structures, we create a dictionary of lists where lists serve as values for immutable keys.

Basic Understanding

Dictionary Syntax

Dictionaries use curly braces {} ?

car_dict = {"Brand": "AUDI", "Model": "A4"}
print(car_dict)

{'Brand': 'AUDI', 'Model': 'A4'}

List Syntax

Lists use square brackets [] ?

details = ["Name", "age", "gender", "qualification"]
print(details)

['Name', 'age', 'gender', 'qualification']

Why Lists Cannot Be Keys

Dictionary keys must be immutable (unchangeable). Lists are mutable, so they cannot serve as keys ?

# This will raise an error
invalid_dict = {["Name", "Age"]: "RAVI"}  # TypeError: unhashable type: 'list'

Method 1: Direct Assignment

Create an empty dictionary and assign lists as values to string keys ?

student_data = {}
student_data["Names"] = ["RAM", "RAVI", "TARUN", "MOHAN"]
student_data["Ages"] = [22, 23, 18, 27]
print(student_data)

{'Names': ['RAM', 'RAVI', 'TARUN', 'MOHAN'], 'Ages': [22, 23, 18, 27]}

Method 2: Using defaultdict

The defaultdict automatically creates missing keys with default list values ?

from collections import defaultdict

data_pairs = [("Name", "Arjun"), ("Age", 22), ("Age", 23), ("Age", 28), ("Name", "RAVI"), ("Name", "ADITYA")]
grouped_data = defaultdict(list)

for key, value in data_pairs:
    grouped_data[key].append(value)

print(dict(grouped_data))

{'Name': ['Arjun', 'RAVI', 'ADITYA'], 'Age': [22, 23, 28]}

Method 3: Using setdefault()

The setdefault() method returns a key's value or creates the key with a default value if missing ?

range_dict = {}
numbers = [23, 24, 28]

for num in numbers:
    for i in range(num, num + 3):
        range_dict.setdefault(i, []).append(num)

print(range_dict)

{23: [23], 24: [23, 24], 25: [23, 24], 26: [24], 27: [], 28: [28], 29: [28], 30: [28]}

Method 4: Dictionary Comprehension

Create a dictionary of lists using comprehension for cleaner syntax ?

categories = ["fruits", "colors", "animals"]
items_dict = {category: [] for category in categories}

# Add items to each category
items_dict["fruits"].extend(["apple", "banana", "orange"])
items_dict["colors"].extend(["red", "blue", "green"])
items_dict["animals"].extend(["cat", "dog", "bird"])

print(items_dict)

{'fruits': ['apple', 'banana', 'orange'], 'colors': ['red', 'blue', 'green'], 'animals': ['cat', 'dog', 'bird']}

Comparison

Conclusion

Dictionary of lists is useful for grouping related data. Use defaultdict for dynamic grouping, direct assignment for simple cases, and setdefault() when you need conditional key creation.