code: def social_media(username, platform='twitter', size='medium'):
          
          return 'https://avatars.io/{platform}/{username}/{size}'.format(
              platform=platform, username=username, size=size)
docs: Return avatar URL at social media.
          Visit https://avatars.io for more information.
  
          :param username: The username of the social media.
          :param platform: One of facebook, instagram, twitter, gravatar.
          :param size: The size of avatar, one of small, medium and large.