def listener(messages):
    for m in messages:
        cid = m.chat.id
        mensaje = ""
        if m.content_type == 'text':
            if cid > 0:
                mensaje = '' + str(m.chat.first_name) + "[" + str(cid) + "]: " + m.text
            else:
                mensaje = '' + str(m.from_user.first_name) + "[" + str(cid) + "]: " + m.text
        with open('/path/to/chats.json', 'r') as f:
            chats = json.load(f)
        if not m.chat.id in chats['chats']:
            chats['chats'].append(m.chat.id)
            with open('/path/to/chats.json', 'w') as f:
                json.dump(chats, f)

bot.set_update_listener(listener)