Python | Extract specific keys from dictionary

We have a lot of variations and applications of dictionary containers in Python and sometimes, we wish to perform a filter of keys in a dictionary, i.e extracting just the keys which are present in the particular container. Let’s discuss certain ways in which this can be performed. 

Method 1: Extract specific keys from dictionary using dictionary comprehension + items()

This problem can be performed by reconstruction using the keys extracted through the items function that wishes to be filtered and the dictionary function makes the desired dictionary.

Python3

# initializing dictionary
test_dict = {'nikhil': 1, "akash": 2, 'akshat': 3, 'manjeet': 4}

# printing original list
print("The original dictionary : " + str(test_dict))

# Using dictionary comprehension + items()
# Extracting specific keys from dictionary
res = {key: test_dict[key] for key in test_dict.keys()
       & {'akshat', 'nikhil'}}

# print result
print("The filtered dictionary is : " + str(res))

Output : 

The original dictionary : {'manjeet': 4, 'akshat': 3, 'akash': 2, 'nikhil': 1}
The filtered dictionary is : {'akshat': 3, 'nikhil': 1}

The time complexity of the given code is O(n), where n is the number of elements in the dictionary. 

The space complexity of the given code is O(k), where k is the number of keys extracted from the dictionary.

Method 2: Extract specific keys from the dictionary using dict()

The dict() function can be used to perform this task by converting the logic performed using list comprehension into a dictionary. 

Python3

# initializing dictionary
test_dict = {'nikhil': 1, "akash": 2, 'akshat': 3, 'manjeet': 4}

# printing original list
print("The original dictionary : " + str(test_dict))

# Using dict()
# Extracting specific keys from dictionary
res = dict((k, test_dict[k]) for k in ['nikhil', 'akshat']
           if k in test_dict)

# print result
print("The filtered dictionary is : " + str(res))

Output : 

The original dictionary : {'manjeet': 4, 'akshat': 3, 'akash': 2, 'nikhil': 1}
The filtered dictionary is : {'akshat': 3, 'nikhil': 1}

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.

Method 3: Extract specific keys from dictionary using filter()

This method involves using the filter() function to filter the keys in the dictionary:

Python3

#initializing dictionary
test_dict = {'nikhil': 1, "akash": 2, 'akshat': 3, 'manjeet': 4}

#printing original list
print("The original dictionary : " + str(test_dict))

#Using filter()
#Extracting specific keys from dictionary
keys_to_extract = ['nikhil', 'akshat']
res = dict(filter(lambda item: item[0] in keys_to_extract, test_dict.items()))

#print result
print("The filtered dictionary is : " + str(res))
#This code is contributed by Edula Vinay Kumar Reddy

Output

The original dictionary : {'nikhil': 1, 'akash': 2, 'akshat': 3, 'manjeet': 4}
The filtered dictionary is : {'nikhil': 1, 'akshat': 3}

Time complexity: O(n)
Auxiliary Space: O(n) for storing the result

Method 4: Using a for loop and a conditional statement 

Initialize the dictionary:

1. Define the list of keys to be extracted.
2. Initialize an empty dictionary to store the extracted keys and their values.
3. Use a for loop and a conditional statement to check if each key in the dictionary is in the list of keys to be extracted. If it is, add the key-value pair to the extracted dictionary.
4. Print the extracted dictionary.

Python3

# initializing dictionary
test_dict = {'nikhil': 1, "akash": 2, 'akshat': 3, 'manjeet': 4}

# printing original list
print("The original dictionary is:", test_dict)

# defining keys to extract
keys_to_extract = ['nikhil', 'akshat']

# initializing an empty dictionary for extracted keys
extracted_dict = {}

# extracting keys using a for loop and conditional statement
for key, value in test_dict.items():
    if key in keys_to_extract:
        extracted_dict[key] = value

# printing the extracted dictionary
print("The extracted dictionary is:", extracted_dict)

Output

The original dictionary is: {'nikhil': 1, 'akash': 2, 'akshat': 3, 'manjeet': 4}
The extracted dictionary is: {'nikhil': 1, 'akshat': 3}

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