from telegram.ext import Updater, CommandHandler

def start(bot, update):
    update.message.reply_text('Hello World!')

def hello(bot, update):
    update.message.reply_text(
        'Hello {}'.format(update.message.from_user.first_name))

updater = Updater('YOUR TOKEN HERE')

updater.dispatcher.add_handler(CommandHandler('start', start))
updater.dispatcher.add_handler(CommandHandler('hello', hello))

updater.start_polling()
try:
    # this call to non-existing function may cause dispatcher.stop() to be called
    # before dispatcher.start() had time to set self.running=True,
    # thus stop() does nothing and dispatcher continues to run.
    CALL_TO_NON_EXISTING_FUNC()
finally:
    updater.stop()