app.py

import os
import slack
from flask import Flask, request
from slackeventsapi import SlackEventAdapter
from uuid import uuid4

client_id = os.environ["SLACK_CLIENT_ID"]
client_secret = os.environ["SLACK_CLIENT_SECRET"]
signing_secret = os.environ["SLACK_SIGNING_SECRET"]
state = str(uuid4())
# Scopes needed for this app
oauth_scope = ", ".join(["chat:write", "channels:read", "channels:join", "channels:manage"])

# Create a dictionary to represent a database to store our token
token_database = {}
global_token = ""

app = Flask(__name__)

# Route to kick off Oauth flow
@app.route("/begin_auth", methods=["GET"])
def pre_install():
  return f'<a href="https://slack.com/oauth/v2/authorize?scope={ oauth_scope }&client_id={ client_id }&state={state}"><img alt=""Add to Slack"" height="40" width="139" src="https://platform.slack-edge.com/img/add_to_slack.png" srcset="https://platform.slack-edge.com/img/add_to_slack.png 1x, https://platform.slack-edge.com/img/add_to_slack@2x.png 2x" /></a>'

# Route for Oauth flow to redirect to after user accepts scopes
@app.route("/finish_auth", methods=["GET", "POST"])
def post_install():
  # Retrieve the auth code and state from the request params
  auth_code = request.args["code"]
  received_state = request.args["state"]

  # Token is not required to call the oauth.v2.access method
  client = slack.WebClient()
  
  # verify state received in params matches state we originally sent in auth request
  if received_state == state:
    # Exchange the authorization code for an access token with Slack
    response = client.oauth_v2_access(
        client_id=client_id,
        client_secret=client_secret,
        code=auth_code
    )
  else:
    return "Invalid State"


  # Save the bot token and teamID to a database
  # In our example, we are saving it to dictionary to represent a DB  
  teamID = response["team"]["id"]
  token_database[teamID] = response["access_token"]
  # Also save the bot token in a global variable so we don't have to 
  # do a database lookup on each WebClient call
  global global_token
  global_token = response["access_token"]

  # See if "the-welcome-channel" exists. Create it if it doesn't. 
  channel_exists()

  # Don't forget to let the user know that auth has succeeded!
  return "Auth complete!"

# verifies if "the-welcome-channel" already exists
def channel_exists():
  client = slack.WebClient(token=global_token)

  # grab a list of all the channels in a workspace
  clist = client.conversations_list()
  exists = False
  for k in clist["channels"]:
    # look for the channel in the list of existing channels
    if k["name"] == "the-welcome-channel":
      exists = True
      break
  if exists == False:
    # create the channel since it doesn't exist
    create_channel()

# creates a channel named "the-welcome-channel"
def create_channel():
  client = slack.WebClient(token=global_token)
  resp = client.conversations_create(name="the-welcome-channel")

# Bind the Events API route to your existing Flask app by passing the server
# instance as the last param, or with `server=app`.
slack_events_adapter = SlackEventAdapter(signing_secret, "/slack/events", app)

# Create an event listener for "member_joined_channel" events
# Sends a DM to the user who joined the channel
@slack_events_adapter.on("member_joined_channel")
def member_joined_channel(event_data):
  user = event_data["event"]["user"]
  channelid = event_data["event"]["channel"]
  teamID = event_data["team_id"]

  # look up the token in our "database"
  token = token_database[teamID]
  
  # In case the app doesn't have access to the oAuth Token
  if token is None:
    print("ERROR: Autenticate the App!")
    return
  client = slack.WebClient(token=token)

  # Use conversations.info method to get channel name for DM msg
  info = client.conversations_info(channel=channelid)
  msg = f'Welcome! Thanks for joining {info["channel"]["name"]}'
  client.chat_postMessage(channel=user, text=msg)