Navigation Loop with Modulo

You can use the modulo operator to wrap around items. This will loop back in both directions.

# Previous item
current_index = (current_index + items_length - 1) % items_length

# Next item
current_index = (current_index + 1) % items_length

The most popular use would be handling navigation in image viewers.

Here is a simple visualization.

Below is the full code for this demo. It uses only the stock Python libraries.

#!/usr/bin/python3
import curses
from curses import wrapper

def main(stdscr):
    images_len = 5
    cur_index = 0

    instructions = [
        "------Instructions------",
        "Left / Right - Navigate",
        "CTRL + C - Quit",
        "------------------------",
    ]

    for i, instruction in enumerate(instructions):
        stdscr.addstr(i, 0, instruction)

    for i in range(images_len):
        stdscr.addstr(len(instructions), i * 5, str(i))

    stdscr.addstr(len(instructions) + 1, cur_index * 5, f"\u2191")
    try:
        while True:
            char = stdscr.getch()
            stdscr.move(len(instructions) + 1, 0)
            stdscr.clrtoeol()
            
            if char == ord('q'):
                break

            # Loop through items
            match char:
                case curses.KEY_LEFT:
                    cur_index = (cur_index + images_len - 1) % images_len
                case curses.KEY_RIGHT:
                    cur_index = (cur_index + 1) % images_len
            
            stdscr.addstr(len(instructions) + 1, cur_index * 5, f"\u2191")

    except KeyboardInterrupt:
        return
    
if __name__ == "__main__":
    wrapper(main)