Ways to create a dictionary of Lists - Python

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:

# Creating an empty dictionary
d = {}

# Adding list as value
d["1"] = [1, 2]
d["2"] = ["Geeks", "For", "Geeks"] 

print(d)

{'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:

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:

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)

{'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:

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)

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:

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)

{'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:

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)

{'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.