import os
import slack

@slack.RTMClient.run_on(event='message')
def say_hello(**payload):
data = payload['data']
web_client = payload['web_client']
rtm_client = payload['rtm_client']
if 'Hello' in data['text']:
channel_id = data['channel']
thread_ts = data['ts']
user = data['user']

web_client.chat_postMessage(
channel=channel_id,
text=f"Hi <@{user}>!",
thread_ts=thread_ts
)

slack_token = os.environ["SLACK_API_TOKEN"]
rtm_client = slack.RTMClient(token=slack_token)
rtm_client.start()