Python – Append List every Nth index

When you need to insert elements from one list at every Nth index position of another list, you can use enumerate() to track indices and conditionally insert elements.

Basic Example

Here's how to append a list every Nth index ?

numbers = [13, 27, 48, 12, 21, 45, 28, 19, 63]
append_list = ['P', 'Y', 'T']
N = 3

print("Original list:")
print(numbers)
print(f"Append list: {append_list}")
print(f"N = {N}")

result = []

for index, element in enumerate(numbers):
    if index % N == 0:
        for item in append_list:
            result.append(item)
    result.append(element)

print("Result:")
print(result)

Original list:
[13, 27, 48, 12, 21, 45, 28, 19, 63]
Append list: ['P', 'Y', 'T']
N = 3
Result:
['P', 'Y', 'T', 13, 27, 48, 'P', 'Y', 'T', 12, 21, 45, 'P', 'Y', 'T', 28, 19, 63]

How It Works

The algorithm works as follows:

• enumerate() provides both index and element from the original list
• index % N == 0 checks if the current position is divisible by N
• When true, all elements from append_list are added before the current element
• The current element is always added to maintain the original sequence

Alternative Using List Comprehension

You can achieve the same result with a more compact approach ?

numbers = [13, 27, 48, 12, 21, 45, 28, 19, 63]
append_list = ['P', 'Y', 'T']
N = 3

result = []
for i, num in enumerate(numbers):
    if i % N == 0:
        result.extend(append_list)
    result.append(num)

print("Result using extend():")
print(result)

Result using extend():
['P', 'Y', 'T', 13, 27, 48, 'P', 'Y', 'T', 12, 21, 45, 'P', 'Y', 'T', 28, 19, 63]

Different N Values

Here's how the output changes with different N values ?

numbers = [1, 2, 3, 4, 5, 6]
append_list = ['X']

for N in [1, 2, 3]:
    result = []
    for i, num in enumerate(numbers):
        if i % N == 0:
            result.extend(append_list)
        result.append(num)
    
    print(f"N={N}: {result}")

N=1: ['X', 1, 'X', 2, 'X', 3, 'X', 4, 'X', 5, 'X', 6]
N=2: ['X', 1, 2, 'X', 3, 4, 'X', 5, 6]
N=3: ['X', 1, 2, 3, 'X', 4, 5, 6]

Conclusion

Use enumerate() with modulo operator to insert elements at every Nth index. The extend() method provides a cleaner way to add multiple elements at once compared to nested loops.