How to Merge Sorted Chunk Files in Python

import heapq


def merge_sorted_chunks(chunks, output_path):
    """Merge multiple sorted iterables into single sorted output file."""
    with open(output_path, "w") as out_f:
        # Open all chunk files
        handles = [open(chunk, "r") for chunk in chunks]
        try:
            # Heap of (value, index) tuples; value is current line
            heap = []
            for idx, fh in enumerate(handles):
                line = fh.readline()
                if line:
                    heap.append((line.strip(), idx))
            heapq.heapify(heap)

            while heap:
                # Pop smallest value
                value, idx = heapq.heappop(heap)
                out_f.write(value + "\n")

                # Read next line from the same chunk
                nxt = handles[idx].readline()
                if nxt:
                    heapq.heappush(heap, (nxt.strip(), idx))
        finally:
            for fh in handles:
                fh.close()


if __name__ == "__main__":
    # Demo: write three sorted chunk files
    chunk_files = []
    for i, data in enumerate([[1, 5, 9], [2, 6, 10], [3, 7, 11]]):
        name = f"chunk_{i}.txt"
        with open(name, "w") as f:
            for num in data:
                f.write(f"{num}\n")
        chunk_files.append(name)

    # Merge and display the result
    output = "merged.txt"
    merge_sorted_chunks(chunk_files, output)
    print(f"Merged output from {chunk_files}:")
    with open(output) as f:
        print(f.read().strip())