code: def request_captcha(size=None, width=None, height=None, ttl=None):
      
      params = {
          'size': size,
          'width': width,
          'height': height,
          'ttl': ttl,
      }
      params = {k: v for k, v in params.items() if v is not None}
  
      response = leancloud.client.get('/requestCaptcha', params)
      content = response.json()
      return Captcha(content['captcha_token'], content['captcha_url'])
docs: 请求生成新的图形验证码
      :return: Captcha