code: def check_credentials(self):
          
          if self.username is None or self.password is None:
              raise DistlibException('username and password must be set')
          pm = HTTPPasswordMgr()
          _, netloc, _, _, _, _ = urlparse(self.url)
          pm.add_password(self.realm, netloc, self.username, self.password)
          self.password_handler = HTTPBasicAuthHandler(pm)
docs: Check that ``username`` and ``password`` have been set, and raise an
          exception if not.