code: def validate_username(username):
      
  
      # Check username looks ok
  
      if not username.islower():
          raise UsernameInvalid(six.u('Username must be all lowercase'))
      if len(username) < 2:
          raise UsernameInvalid(six.u('Username must be at least 2 characters'))
      if not username_re.search(username):
          raise UsernameInvalid(settings.USERNAME_VALIDATION_ERROR_MSG)
  
      return username
docs: Validate the new username. If the username is invalid, raises
      :py:exc:`UsernameInvalid`.
  
      :param username: Username to validate.