How Can I Efficiently Interlace Two Lists of Potentially Different Lengths in Python?

Pythonic Interlacing: Combining Lists Alternately

In Python, interlacing two lists means creating a new list that alternates elements from both lists. To achieve this, consider the following scenarios:

Length-Matched Lists

If both lists have an equal number of elements, a simple solution is to use slicing:

list1 = ['f', 'o', 'o']
list2 = ['hello', 'world']
result = [None] * (len(list1) + len(list2))
result[::2] = list1
result[1::2] = list2
print(result)

This will produce the desired output:

['f', 'hello', 'o', 'world', 'o']

Length-Mismatched Lists

When the input lists have different lengths, additional logic is required:

Leaving Excess Elements at the End

To leave any excess elements from the longer list at the end, use this approach:

def interlace(list1, list2):
    result = []
    i, j = 0, 0  # indices for list1 and list2

    while i < len(list1) and j < len(list2):
        result.append(list1[i])
        result.append(list2[j])
        i += 1
        j += 1

    # Add remaining elements from the longer list
    result.extend(list1[i:] if len(list1) > len(list2) else list2[j:])

    return result

Interspersing Elements Evenly

To spread out excess elements evenly within the interlaced list, use this method:

def interlace_evenly(list1, list2):
    shorter_list = list1 if len(list1) < len(list2) else list2
    longer_list = list1 if len(list1) > len(list2) else list2

    result = []

    # Intersperse elements of the shorter list
    for i in range(len(shorter_list)):
        result.append(shorter_list[i])
        result.append(longer_list[i % len(longer_list)])

    # Add remaining elements from the longer list
    result.extend(longer_list[len(shorter_list):])

    return result

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