How to route late-arriving data to a side output in Python

from collections import defaultdict

def late_arriving_side_output(events, late_threshold_ts):
    """
    Mock a streaming pipeline that separates late-arriving data events
    into a side output list (e.g., for dead-letter analysis).

    events: list of (timestamp, data) tuples, timestamps as ints.
    late_threshold_ts: int — events with timestamp < this are considered late.
    Returns: (main_output, side_output_list) — two lists.
    """
    main_output = []
    side_output_list = []

    for ts, data in events:
        if ts < late_threshold_ts:
            side_output_list.append((ts, data))
        else:
            main_output.append((ts, data))

    return main_output, side_output_list


if __name__ == "__main__":
    events = [
        (100, "on-time-1"),
        (95, "late-1"),
        (120, "on-time-2"),
        (90, "late-2"),
        (130, "on-time-3"),
    ]
    threshold = 100

    main, side = late_arriving_side_output(events, threshold)
    print("Main output:", main)
    print("Side output list:", side)