test.py

import telegram.ext as tg
from telegram import Update, TelegramError
from telegram.ext import run_async, CallbackContext, CommandHandler, DispatcherHandlerStop, Dispatcher


def process_update(self, update):

    for group in self.groups:
        try:
            for handler in (x for x in self.handlers[group] if x.check_update(update)):
                # I think this line is the problem:
                handler.handle_update(update, self, handler.check_update(update))
                break

        # Stop processing with any other handler.
        except DispatcherHandlerStop:
            self.logger.debug("Stopping further handlers due to DispatcherHandlerStop")
            break

        # Dispatch any error.
        except TelegramError as te:
            self.logger.warning("A TelegramError was raised while processing the Update")

            try:
                self.dispatch_error(update, te)
            except DispatcherHandlerStop:
                self.logger.debug("Error handler stopped further handlers")
                break
            except Exception:
                self.logger.exception("An uncaught error was raised while handling the error")

        # Errors should not stop the thread.
        except Exception:
            self.logger.exception("An uncaught error was raised while processing the update")

@run_async
def start(update: Update, context: CallbackContext):
    if update.effective_chat.type == "private":
        args = context.args
        first_name = update.effective_user.first_name
        update.effective_message.reply_text("Hello " + first_name)
    else:
        update.effective_message.reply_text("You started this bot in a group.")  # START_IN_GROUP



if __name__ == '__main__':
    TOKEN = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
    WORKERS=8
    updater = tg.Updater(TOKEN, workers=WORKERS, use_context=True)
    dispatcher = updater.dispatcher
    start_handler = CommandHandler("start", start)
    dispatcher.add_handler(start_handler)
    Dispatcher.process_update = process_update
    updater.start_polling(timeout=15, read_latency=4)
    updater.idle()