Initializing dictionary with Empty Lists in Python (original) (raw)

Last Updated : 20 Dec, 2023

In Python, it’s common to encounter scenarios where you need to store lists in a dictionary. Often, this involves checking if a key exists and then creating a list for that key. However, a more efficient approach is to initialize the dictionary keys with empty lists from the start. Let’s explore some methods to accomplish this.

How to Initialize a Dictionary with an Empty List

It is a good practice to initialize a dictionary with empty lists in Python. This helps in working with dynamic data structures. This also helps in appending items to a list associated with a key.

Initializing an empty list while creating a dictionary also eases the process of later creating a list while checking for key availability.

We will learn various methods to initialize an empty list dictionary.

Initialize a Dictionary of Empty Lists using Dictionary Comprehension

Dictionary comprehension is the most sought method to do this initialization. In this method, we create the no. of keys we require and then initialize the empty list as we keep on creating the keys, to facilitate the append operation afterward without an error.

Python3

new_dict = {new_list: [] for new_list in range ( 4 )}

print ( "New dictionary with empty lists as keys : " + str (new_dict))

Output

New dictionary with empty lists as keys : {0: [], 1: [], 2: [], 3: []}

Time complexity: O(n), where n is the number of key-value pairs in the dictionary.
Auxiliary space: O(n), to store the keys and values in dictionary.

Create Empty List of Dictionaries using **fromkeys()

Using fromkeys() you can create a dictionary from the given sequence of keys and values.

Python3

alphabets = { 'a' , 'b' , 'c' }

number = 10

dictionary = dict .fromkeys(alphabets, number)

print (dictionary)

Output

{'b': 10, 'a': 10, 'c': 10}

Create Empty List of Dictionaries using defaultdict

This is the most pythonic way and error-free way to use dictionary key without needing to initialize its value. It has to be told the type of default container of all its keys and then dictionary automatically evaluates the operations and structures accordingly.

Python3

from collections import defaultdict

new_dict = defaultdict( list )

new_dict[ 0 ].append( 'GeeksforGeeks' )

print ( "New dictionary created : " + str ( dict (new_dict)))

Output

New dictionary created : {0: ['GeeksforGeeks']}

Python Initialize Dictionary using setdefault

This can be used to perform by specifying key-value pairs within a comprehension. This method obviates the need to import a module as is required in the above method.

Python3

new_dict = {}

[new_dict.setdefault(x, []) for x in range ( 4 )]

new_dict[ 0 ].append( 'GeeksforGeeks' )

print ( "New dictionary created : " + str ( dict (new_dict)))

Output

New dictionary created : {0: ['GeeksforGeeks'], 1: [], 2: [], 3: []}

Initialize Dictionary with Empty List using built-ins: dict and zip

The built-in functions dict, and zip in conjunction with list comprehension can achieve the desired result.

Python3

keys = range ( 4 )

new_dict = dict ( zip (keys, ([] for _ in keys)))

print (new_dict)

new_dict[ 0 ].append( 'GeeksforGeeks' )

print ( "New dictionary created : " + str ( dict (new_dict)))

Output

{0: [], 1: [], 2: [], 3: []} New dictionary created : {0: ['GeeksforGeeks'], 1: [], 2: [], 3: []}

Initialize Dictionary with Empty List using Itertools

Here is an example of using the itertools module to initialize a dictionary with empty lists using itertools.

Python3

import itertools

keys = [ 'a' , 'b' , 'c' ]

values = list (itertools.repeat([], len (keys)))

key_value_pairs = zip (keys, values)

my_dict = dict (key_value_pairs)

print (my_dict)

Output

{'a': [], 'b': [], 'c': []}

**Explanation:

We have covered various methods to initialize a dictionary with empty list. Using lists inside a dictionary is very useful, and you should try to make the dictionary with empty list so that you can use it in future without worrying to add new list.

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