send_message.py

import random
import time
import json
import threading
import click


def send_message(msg, async=True):
    """Logs the message to an output stream.

    If async is true, use a background thread

    Based on
    http://sebastiandahlgren.se/2014/06/27/running-a-method-as-a-background-thread-in-python/
    """

    def send(msg=msg):
        try:
            # Simulates a system where the IO could take up to 5 seconds
            time.sleep(random.random() * 5)
            print(f"Sending {msg}")  # IO
        except Exception:
            pass  # swallow any and all exceptions

    if async:
        thread = threading.Thread(target=send)
        thread.daemon = True
        thread.start()
    else:
        send()


@click.command()
@click.option('--async/--sync', is_flag=True, default=True)
def main(async):
    click.echo("Async..." if async else "Sync...")
    for i in range(10):
        send_message(json.dumps({'a': i}), async=async)

    print("I'm done logging, start the heavy processing")
    time.sleep(5)
    print("I'm done processing")


if __name__ == "__main__":
    main()

If the main process exits before the threads, any remaining threads are "orphaned" and may not complete. To avoid this, set thread.daemon = False to allow all threads to run until completion.