如何检查python中是否存在JSON键/对象

Question

我对Python很陌生。除了下面的代码之外，还有什么可以检查python中是否存在特定的JSON对象吗？承认我下面的代码不是很好的实践，所以需要知道哪种方法更易于检查和维护？

以下是JSON响应：

[
  {
    "MessageId": "250e37a8-d779-48a1-9941-84219a82513e",
    "ReceiptHandle": "AQEBjualXe2ywqTgIVmCNI5sKj7r48werf84HHA2BWZimwiEXLFxA/MiPBclK048NZBtOnM3dSDfoiwwqoNTPxTRz+IChd8McziweCxHX6texjAOi/MyAQjCWP+2hJPoxzQgXx9kjnKbepKlcgxhpOiQZe6WiSIq0dXwHHXSA7SP0g9NIR/dU38b+wmo0m2q7MNVfSct967EKF49wow9RHyFMO8iD8fH93PYT9om5NdUha3dvkWnisKcfuO5pZY3LLXPAnuZT/VfqxJjmPqb98iepBfqFb6SpM/02IVSql81XKJEbMBc4zPHp/Uace6e4UDGsn/hPCVsqQsTzrbKCR+ovpkhXipWwTYSlgsLe/o43k0UxhCN8eKhg835KuUkskA3T8C5Q6v6xgznlR7JJuhZpg==",
    "MD5OfBody": "bbdc5fdb8be7251f5c910905db994bab",
    "Body": "Information about current NY Times fiction bestseller for week of 12/11/2016.",
    "Attributes": {
      "SentTimestamp": "1553851566164"
    },
    "MD5OfMessageAttributes": "d25a6aea97eb8f585bfa92d314504a92",
    "MessageAttributes": {
      "Author": {
        "StringValue": "John Grisham",
        "DataType": "String"
      },
      "Title": {
        "StringValue": "The Whistler",
        "DataType": "String"
      },
      "WeeksOn": {
        "StringValue": "6",
        "DataType": "Number"
      }
    }
  }
]

下面是我要检查的python代码：

if 'Messages' in response:
    message = response['Messages'][0]
    receipt_handle = message['ReceiptHandle']

    sqs.delete_message(
        QueueUrl=queue_url,
        ReceiptHandle=receipt_handle
    )
    print('Received and deleted message: %s' % message)
else:
    print('Message not received yet')

请让我知道以上代码是否是良好的做法。

Answer 1

您可以签出的另一个选项，也是我在使用JSON数据时实际喜欢的选项，是从JSON数据创建一个对象并使用hasattr方法。这将防止重新开始使用try-except块，也可以使代码更易于理解。使用数据的示例如下：

data= '''
  {
"MessageId": "250e37a8-d779-48a1-9941-84219a82513e",
"ReceiptHandle": "AQEBjualXe2ywqTgIVmCNI5sKj7r48werf84HHA2BWZimwiEXLFxA/MiPBclK048NZBtOnM3dSDfoiwwqoNTPxTRz+IChd8McziweCxHX6texjAOi/MyAQjCWP+2hJPoxzQgXx9kjnKbepKlcgxhpOiQZe6WiSIq0dXwHHXSA7SP0g9NIR/dU38b+wmo0m2q7MNVfSct967EKF49wow9RHyFMO8iD8fH93PYT9om5NdUha3dvkWnisKcfuO5pZY3LLXPAnuZT/VfqxJjmPqb98iepBfqFb6SpM/02IVSql81XKJEbMBc4zPHp/Uace6e4UDGsn/hPCVsqQsTzrbKCR+ovpkhXipWwTYSlgsLe/o43k0UxhCN8eKhg835KuUkskA3T8C5Q6v6xgznlR7JJuhZpg==",
"MD5OfBody": "bbdc5fdb8be7251f5c910905db994bab",
"Body": "Information about current NY Times fiction bestseller for week of 12/11/2016.",
"Attributes": {"SentTimestamp": "1553851566164"},
"MD5OfMessageAttributes": "d25a6aea97eb8f585bfa92d314504a92",
"MessageAttributes": {"Author": {"StringValue": "John Grisham","DataType": "String"},"Title": {"StringValue": "The Whistler","DataType": "String"},"WeeksOn": {"StringValue": "6","DataType": "Number"}}
  } '''

import json

class Response:

    def __init__(self, data):
        self.__dict__ = json.loads(data)

response = Response(data)

if hasattr(response , 'MessageId'):
    receipt_handle = response.ReceiptHandle
    print("Received and deleted message: %s" % response.MessageId)

else:
    print('Message not received yet')

输出：

Received and deleted message: 250e37a8-d779-48a1-9941-84219a82513e

Answer 2

前面已经提到过，示例json不包含键Messages。我觉得你的代码看起来不错。但是，如果没有键Messages的json非常罕见，那么我将使用try-except块。

try:
    message = response['Messages'][0]
    receipt_handle = message['ReceiptHandle']

    sqs.delete_message(
        QueueUrl=queue_url,
        ReceiptHandle=receipt_handle
    )
    print('Received and deleted message: %s' % message)
except KeyError:
    print('Message not received yet')

每次您获得“正确”的json时，这将更快。但是当您得到一个缺少键的json时速度会慢一些。所以也许你需要弄清楚没有钥匙的情况是否经常发生。

但这取决于用例。我的答案只是我自己的观点和类似用例的经验

Answer 3

由于响应是dict的list：

j_data = [{'MessageId': '250e37a8-d779-48a1-9941-84219a82513e',
           'ReceiptHandle': 'AQEBjualXJJuhZpg==', 'MD5OfBody': 'bbdc5f905db994bab',
           'Body': 'Information about current NY Times fiction bestseller for week of 12/11/2016.',
           'Attributes': {'SentTimestamp': '1553851566164'},
           'MD5OfMessageAttributes': 'd25a6aea97eb8f585bfa92d314504a92',
           'MessageAttributes': {'Author': {'StringValue': 'John Grisham', 'DataType': 'String'},
                                 'Title': {'StringValue': 'The Whistler', 'DataType': 'String'},
                                 'WeeksOn': {'StringValue': '6', 'DataType': 'Number'}}}
            ]



for data in j_data:
    try:
        if 'MessageId' in data:
            message = data['MessageId']
            receipt_handle = data['ReceiptHandle']
            sentTimeStamp = data['Attributes']['SentTimestamp']
            print(message)
            print(receipt_handle)
            print(sentTimeStamp)
    except KeyError:
        print("Some custom message here")

输出：

250e37a8-d779-48a1-9941-84219a82513e
AQEBjualXJJuhZpg==
1553851566164

编辑：

另一种方法是在访问之前检查每个密钥，即（从响应中删除ReceiptHandleelem）：

j_data = [{'MessageId': '250e37a8-d779-48a1-9941-84219a82513e',
           'MD5OfBody': 'bbdc5f905db994bab',
           'Body': 'Information about current NY Times fiction bestseller for week of 12/11/2016.',
           'Attributes': {'SentTimestamp': '1553851566164'},
           'MD5OfMessageAttributes': 'd25a6aea97eb8f585bfa92d314504a92',
           'MessageAttributes': {'Author': {'StringValue': 'John Grisham', 'DataType': 'String'},
                                 'Title': {'StringValue': 'The Whistler', 'DataType': 'String'},
                                 'WeeksOn': {'StringValue': '6', 'DataType': 'Number'}}}
            ]



for data in j_data:
    try:
        if 'MessageId' in data:
            message = data['MessageId']
            print(message)
        if 'ReceiptHandle' in data:
            receipt_handle = data['ReceiptHandle']
            print(receipt_handle)
        if 'Attributes' in data:
            sentTimeStamp = data['Attributes']['SentTimestamp']
            print(sentTimeStamp)
    except KeyError:
        print("Some custom message here")

输出：

250e37a8-d779-48a1-9941-84219a82513e
1553851566164