Python | Clearing list as dictionary value

Clearing a list is a common problem and solution to it has been discussed many times. But sometimes, we don’t have a native list but list is a value to dictionary key. Clearing it is not as easy as clearing an original list. Let’s discuss certain ways in which this can be done. 

Method #1: Using loop + clear() This is the most generic method in which we can perform this particular function. We just run a loop till the last dictionary key and clear the key’s list value as they occur using clear function. 

Python3

# Python3 code to demonstrate
# clearing list as dict. value
# using loop + clear()

# initializing dict.
test_dict = {"Akash": [1, 4, 3],
             "Nikhil": [3, 4, 1],
             "Akshat": [7, 8]}

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

# using loop + clear()
# clearing list as dict. value
for key in test_dict:
    test_dict[key].clear()

# print result
print("The dictionary after clearing value list : " + str(test_dict))

Output

The original dict : {'Akash': [1, 4, 3], 'Nikhil': [3, 4, 1], 'Akshat': [7, 8]}
The dictionary after clearing value list : {'Akash': [], 'Nikhil': [], 'Akshat': []}

Time Complexity: O(n)
Auxiliary Space: O(1)

Method #2: Using dictionary comprehension We can reduce the lines of code and merge the above functionality using just the dictionary comprehension and clearing the list using the list re-initialization. 

Python3

# Python3 code to demonstrate
# clearing list as dict. value
# using dictionary comprehension

# initializing dict.
test_dict = {"Akash": [1, 4, 3],
             "Nikhil": [3, 4, 1],
             "Akshat": [7, 8]}

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

# using dictionary comprehension
# clearing list as dict. value
test_dict = {key: [] for key in test_dict}

# print result
print("The dictionary after clearing value list : " + str(test_dict))

Output

The original dict : {'Akash': [1, 4, 3], 'Nikhil': [3, 4, 1], 'Akshat': [7, 8]}
The dictionary after clearing value list : {'Akash': [], 'Nikhil': [], 'Akshat': []}

Time complexity: O(n), where n is the number of keys in the dictionary.
Auxiliary space: O(n), as a new dictionary is created with the same number of keys as the original dictionary.

Method #3: Using the fromkeys() method

We can use the fromkeys() method to create a new dictionary with the specified keys and all values set to an empty list.

Python3

test_dict = {"Akash": [1, 4, 3],
             "Nikhil": [3, 4, 1],
             "Akshat": [7, 8]}

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


# Get the keys of the dictionary
keys = test_dict.keys()

# Create a new dictionary with the same keys and all values set to an empty list
test_dict = dict.fromkeys(keys, [])

# print result
print("The dictionary after clearing value list :" + str(test_dict))

# Output: {'Akash': [], 'Nikhil': [], 'Akshat': []}
# This code is contributed by Edula Vinay Kumar Reddy

Output

The original dict :{'Akash': [1, 4, 3], 'Nikhil': [3, 4, 1], 'Akshat': [7, 8]}
The dictionary after clearing value list :{'Akash': [], 'Nikhil': [], 'Akshat': []}

Time Ccomplexity: O(n)
Auxiliary Space: O(n) for getting keys and creating

Method #4: Use the update() method:

The update() method can be used to update the values of a dictionary in-place. It takes a dictionary, or an iterable of key-value pairs, as an argument and updates the dictionary with the key-value pairs from the argument. Here’s an example of using the update() method to clear a list as a value in a dictionary:

In this example, we use the update() method with a dictionary comprehension to clear the lists in the test_dict dictionary. The dictionary comprehension generates a new iterable of key-value pairs, with the keys being the keys from the test_dict dictionary and the values being an empty list. The update() method updates the test_dict dictionary with these key-value pairs, resulting in all lists being cleared.

Python3

# Python code to demonstrate
# clearing list as dict. value
# using the update() method

# Initialize the dictionary
test_dict = {"Akash" : [1, 4, 3],
             "Nikhil" : [3, 4, 1],
             "Akshat" : [7, 8]}

# Print the original dictionary
print("The original dict :", test_dict)

# Use the update() method to clear the lists
test_dict.update((k, []) for k in test_dict)

# Print the updated dictionary
print("The dictionary after clearing value list :", test_dict)

# Output:
# The original dict : {'Akash': [1, 4, 3], 'Nikhil': [3, 4, 1], 'Akshat': [7, 8]}
# The dictionary after clearing value list : {'Akash': [], 'Nikhil': [], 'Akshat': []}
#This code is contributed by Edula Vinay Kumar Reddy

Output

The original dict : {'Akash': [1, 4, 3], 'Nikhil': [3, 4, 1], 'Akshat': [7, 8]}
The dictionary after clearing value list : {'Akash': [], 'Nikhil': [], 'Akshat': []}

Time Complexity: O(n)
Auxiliary Space: O(n)

Method #5 : Using keys() method

Python3

# Python3 code to demonstrate
# clearing list as dict value

# initializing dict.
test_dict = {"Akash" : [1, 4, 3],
            "Nikhil" : [3, 4, 1],
            "Akshat" : [7, 8]}

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

res=dict()
for key in list(test_dict.keys()):
    res[key]=[]


# print result
print("The dictionary after clearing value list : " + str(res))

Output

The original dict : {'Akash': [1, 4, 3], 'Nikhil': [3, 4, 1], 'Akshat': [7, 8]}
The dictionary after clearing value list : {'Akash': [], 'Nikhil': [], 'Akshat': []}

Time Complexity: O(n)
Auxiliary Space: O(n)

Method #6: Using copy() method

You can create a copy of the original dictionary and clear the list values of the copy. This method does not modify the original dictionary and provides an alternative solution. 

The steps are:

• Create a copy of the original dictionary using the copy() method.
• Use a loop to iterate through the keys of the copy dictionary.
• Clear the list values of each key in the copy dictionary using the clear() method.
• Print the resulting copy dictionary.

Python3

# Python3 code to demonstrate
# clearing list as dict value using copy()

# initializing dict.
test_dict = {"Akash" : [1, 4, 3],
            "Nikhil" : [3, 4, 1],
            "Akshat" : [7, 8]}

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

# creating a copy of the original dict
res = test_dict.copy()

# clearing the list values of the copy dict
for key in res:
    res[key].clear()

# print result
print("The dictionary after clearing value list : " + str(res))

Output

The original dict : {'Akash': [1, 4, 3], 'Nikhil': [3, 4, 1], 'Akshat': [7, 8]}
The dictionary after clearing value list : {'Akash': [], 'Nikhil': [], 'Akshat': []}

Time complexity: O(n), where n is the number of keys in the dictionary.
Auxiliary space: O(n), where n is the number of keys in the dictionary.