Python - Clearing list as dictionary value

In Python, you may have a dictionary where values are lists and need to clear all the list values while keeping the keys. There are two main approaches: using the clear() method to empty existing lists in-place, or using dictionary comprehension to assign new empty lists.

Using Loop with clear() Method

The clear() method empties existing lists in-place, preserving the original list objects ?

fruits = {"Apple": [4, 6, 9, 2], "Grape": [7, 8, 2, 1], "Orange": [3, 6, 2, 4]}
print("Original dictionary:", fruits)

# Clear each list in-place
for key in fruits:
    fruits[key].clear()

print("After clearing:", fruits)

Original dictionary: {'Apple': [4, 6, 9, 2], 'Grape': [7, 8, 2, 1], 'Orange': [3, 6, 2, 4]}
After clearing: {'Apple': [], 'Grape': [], 'Orange': []}

Using Dictionary Comprehension

Dictionary comprehension creates a new dictionary with empty lists assigned to each key ?

fruits = {"mango": [4, 6, 9, 2], "pineapple": [7, 8, 2, 1], "cherry": [3, 6, 2, 4]}
print("Original dictionary:", fruits)

# Create new dictionary with empty lists
fruits = {key: [] for key in fruits}

print("After clearing:", fruits)

Original dictionary: {'mango': [4, 6, 9, 2], 'pineapple': [7, 8, 2, 1], 'cherry': [3, 6, 2, 4]}
After clearing: {'mango': [], 'pineapple': [], 'cherry': []}

Comparison

Conclusion

Use clear() for in-place modification when memory efficiency matters. Use dictionary comprehension when you need a completely new dictionary with empty lists.