<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def who_am_i():
 """ Display username from current token and check for validity """

    me = url['me'].format(token)
    r = requests.get(me, params={'client_id': CLIENT_ID})
    r.raise_for_status()
    current_user = r.json()
    logger.debug(me)

    logger.info('Hello {0}!'.format(current_user['username']))
    return current_user