code: def login(self, email=None, password=None):
          
          auth_url = self.url('auth')
          email = email or self._config.get('email')
          password = password or self._config.get('password')
          if password and not email:
              raise Exception('Email must be provided when password is.')
          while True:
              if not email:
                  sys.stdout.write('Email: ')
                  sys.stdout.flush()
                  email = sys.stdin.readline().strip()
                  if not email:
                      print('Goodbye!')
                      sys.exit(1)
              if not password:
                  password = getpass.getpass('Password for {0}: '.format(email))
              response = self.request(auth_url, 'PUT', email=email,
                                      password=password)
              if response.status_code == 201:
                  print('logged in as {0}'.format(email))
                  self.save_prompt(email, password)
                  self.email = email
                  break
              else:
                  print(response.json()['messages'])
                  email = password = None
docs: Login to establish a valid session.