向Alexa Event Gateway发送事件时遇JSON格式错误求助

问题描述 #

我正在为运动传感器开发Alexa Skill，尝试向Alexa Event Gateway发送Alexa.ChangeReport事件时，持续收到Malformed JSON错误。已严格遵循亚马逊《MotionSensor Interface Documentation》中的ChangeReport事件格式编写代码，但仍提示JSON格式有误。目前暂未实现令牌刷新逻辑，但已确认所用令牌有效，求排查思路。

代码示例 #

motion_state=get_motion_state_from_iot()
acc_token = payload['state']['desired']['LWA_Access_Token']
message_id = str(uuid.uuid4())
current_time = datetime.datetime.utcnow().isoformat() + "Z"

event_data = urllib.parse.urlencode(
{  
  "event": {
    "header": {
      "namespace": "Alexa",
      "name": "ChangeReport",
      "messageId": message_id,
      "payloadVersion": "3"
    },
    "endpoint": {
      "scope": {
        "type": "BearerToken",
        "token": acc_token
      },
      "endpointId": endpoint_id
    },
    "payload": {
      "change": {
        "cause": {
          "type": "PHYSICAL_INTERACTION"
        },
        "properties": [
          {
            "namespace": "Alexa.MotionSensor",
            "name": "detectionState",
            "value": motion_state,
            "timeOfSample": current_time,
            "uncertaintyInMilliseconds": 0
          }
        ]
      }
    }
  },
  "context": {
    "properties": [
      {
        "namespace": "Alexa.EndpointHealth",
        "name": "connectivity",
        "value": {
          "value": "OK"
        },
        "timeOfSample": current_time,
        "uncertaintyInMilliseconds": 0
      }
    ]
  }
}
).encode("utf-8")

url = "https://api.eu.amazonalexa.com/v3/events"

headers = {
    'Authorization': f'Bearer {acc_token}',
    'Content-Type': 'application/json'
}

posting = urllib.request.Request(url, event_data, headers, "POST")

try:
    with urllib.request.urlopen(posting) as response:
        lwa_tokens = json.loads(response.read().decode("utf-8"))
        return ("Success!")
        
except HTTPError as http_error:
    return(f"An error occurred: {http_error.read().decode('utf-8')}")

错误信息 #

解码后的错误信息：

{"header":{"namespace":"System","name":"Exception","messageId":"01ca3509-734d-49ba-a89c-40cf93e2c0ae"},"payload":{"code":"INVALID_REQUEST_EXCEPTION","description":"The request was malformed."}}

排查思路 #

• 核心问题：错误使用URL编码处理JSON数据
  代码中使用urllib.parse.urlencode()处理事件数据是错误的，这个方法用于生成URL查询参数格式的字符串（如key=value&key2=value2），但请求头声明了Content-Type: application/json，导致发送的内容不是合法JSON，直接触发格式错误。
  正确做法是用json.dumps()将Python字典序列化为JSON字符串，再编码为utf-8：

  import json
  # 替换原有的urlencode代码段
  event_data = json.dumps({  
    # 保留原事件字典内容不变
  }).encode("utf-8")
  

• 其他潜在检查点

  • 确认motion_state的值是合法枚举：DETECTED或NOT_DETECTED，自定义值会导致请求无效。
  • 检查endpoint_id与Alexa开发者控制台中注册的端点ID完全一致，注意大小写和特殊字符。
  • 验证current_time格式符合ISO 8601标准（例如2024-05-20T12:34:56.789Z），确保无额外字符。
  • 确认令牌权限包含alexa::events:write，这是发送事件的必需权限。

内容的提问来源于stack exchange，提问作者Mohammed Bin Saleem