Python | Initialize a dictionary with only keys from a list

Given a List, the task is to create a dictionary with only keys by using given list as keys. Let’s see the different methods we can do this task. Method #1 : By iterating through list 

Python3

# Python code to initialize a dictionary
# with only keys from a list

# List of keys
keyList = ["Paras", "Jain", "Cyware"]

# initialize dictionary
d = {}

# iterating through the elements of list
for i in keyList:
    d[i] = None

print(d)

Output:

{'Cyware': None, 'Paras': None, 'Jain': None}

Method #2 : Using dictionary comprehension 

Python3

# Python code to initialize a dictionary
# with only keys from a list

# List of Keys
keyList = ["Paras", "Jain", "Cyware"]

# Using Dictionary comprehension
myDict = {key: None for key in keyList}
print(myDict)

Output:

{'Paras': None, 'Jain': None, 'Cyware': None}

Method #3 : Using zip() function 

Python3

# Python code to initialize a dictionary
# with only keys from a list

# List of keys
listKeys = ["Paras", "Jain", "Cyware"]

# using zip() function to create a dictionary
# with keys and same length None value
dct = dict(zip(listKeys, [None]*len(listKeys)))

# print dict
print(dct)

Output:

{'Cyware': None, 'Paras': None, 'Jain': None}

Method #4 : Using fromkeys() method 

Python3

# Python code to initialize a dictionary
# with only keys from a list

# List of keys
Student = ["Paras", "Jain", "Cyware"]

# using fromkeys() method
StudentDict = dict.fromkeys(Student, None)

# printing dictionary
print(StudentDict)

Output:

{'Cyware': None, 'Jain': None, 'Paras': None}

Method #5 : Using the dict.__init__ method:

Here is another approach to creating a dictionary with only keys from a list, using the dict.__init__ method:

Python3

def create_dict(key_list):
    # Initialize an empty dictionary
    d = {}
    # Use the dict.__init__ method to add the keys from the input list to the dictionary
    d.__init__(zip(key_list, [None]*len(key_list)))
    return d

key_list = ["Paras", "Jain", "Cyware"]
print(create_dict(key_list))  # {'Paras': None, 'Jain': None, 'Cyware': None}
#This code is contributed by Edula Vinay Kumar Reddy

Output

{'Paras': None, 'Jain': None, 'Cyware': None}

This approach uses the dict.__init__ method to initialize the dictionary with the keys from the input list and None as the values.

The time complexity of the create_dict function is O(n), where n is the number of keys in the input list. This is because the function iterates through the input list once, and performs a constant number of operations on each iteration.

The space complexity of the create_dict function is also O(n), because the function creates a new dictionary with n keys. Additionally, the function creates a list of n None values to pass as the values for the keys in the dictionary.