Python: Find the indexes of all None items in a given list

Last update on April 19 2025 12:58:11 (UTC/GMT +8 hours)

Indexes of None in List

Write a Python program to find the indexes of all None items in a given list.

Visual Presentation:

Sample Solution:

Python Code:

# Define a function 'relative_order' that finds the indexes of all None items in a list.
def relative_order(lst):
    # Use a list comprehension to iterate over the indexes 'i' and elements in the list 'lst'.
    # For each element, check if it is equal to None, and if it is, include its index 'i' in the result list.
    result = [i for i in range(len(lst)) if lst[i] == None]
    # Return the list of indexes of None items.
    return result

# Define a list 'nums' containing various elements, including None values.
nums = [1, None, 5, 4, None, 0, None, None]

# Print a message indicating the original list.
print("Original list:")
# Print the original list 'nums'.
print(nums)

# Print a message indicating the purpose of the following lines of code.
print("\nIndexes of all None items of the list:")
# Call the 'relative_order' function to find the indexes of all None items in the list 'nums'.
print(relative_order(nums))

Sample Output:

Original list:
[1, None, 5, 4, None, 0, None, None]

Indexes of all None items of the list:
[1, 4, 6, 7]

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Python program to count the number of None values in a list and output their indices.
• Write a Python program to replace all None values in a list with a default value, returning both the new list and the indices where replacements occurred.
• Write a Python program to remove all None elements from a list and produce a separate list containing their original indices.
• Write a Python program to split a list into two lists—one containing None values (with indices) and one with the non-None elements.

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