使用JSON和字典处理Python中的错误 [英] Error handling in Python with JSON and a dictionary

问题描述

我目前拥有一个Python 2.7脚本，可以抓住Facebook并从每个页面捕获一些JSON数据。 JSON数据包含个人信息。 JSON数据的示例如下： -

  {
id：4，
name：Mark Zuckerberg，
first_name：Mark，
last_name：Zuckerberg，
link：http：//www.facebook 。
username：zuck，
gender：male，
locale：en_US
} 
  

JSON值可能因网页而异。上述示例列出了所有可能性，但有时候，诸如用户名之类的值可能不存在，并且可能会遇到JSON数据，例如： -

  {
id：6，
name：Billy Smith，
first_name：Billy，
last_name Smith，
gender：male，
locale：en_US
} 
  

使用这些数据，我想填充数据库表。因此，我的代码如下： -

  results_json = simplejson.loads（scraperwiki.scrape（profile_url））
 for result in results_json：
 profile = dict（）
 try：
 profile ['id'] = int（results_json ['id']）
除了：
 profile ['id'] =
 try：
 profile ['name'] = results_json ['name'] 
除了：
 profile ['name'] = 
 try：
 profile ['first_name'] = results_json ['first_name'] 
除了：
 profile ['first_name'] =
 try： 
 profile ['last_name'] = results_json ['last_name'] 
除了：
 profile ['last_name'] =
 try：
 profile ['link '] = results_json ['link'] 
除了：
个人资料['link'] =
 try：
个人资料['username'] = results_json ['username'] 
除了：
 profile ['username'] = 
 try：
 profile ['gender'] = results_json ['gender'] 
 except：
 profile ['gender'] =
 try：
 profile ['locale'] = results_json ['locale'] 
除了：
 profile ['locale'] =
  

我有这么多尝试/除外的原因是说明当网页上不存在键值时。尽管如此，这似乎是一个非常复杂和混乱的方式来处理这个问题。

如果我删除这些try / exception子句，如果我的刮刀遇到缺少的键，它会返回一个 KeyError 作为KeyError：'username'，我的脚本停止运行。

任何关于更智能和改进的建议方法来处理这些错误，以便在遇到缺少的密钥时，脚本继续。

我已经尝试创建了一个JSON值的列表，并希望通过一个IF子句来遍历它们，但是我无法理解。

解决方案

使用 .get（）方法：

 > ;>> a = {'bar'：'eggs'} 
>>>一个['foo'] 
追溯（最近的最后一次呼叫）：
文件< stdin>，第1行，< module> 
 KeyError：'foo'
>>> a.get（'foo'，'default value'）
'默认值'
>>> a.get（'bar'，'default value'）
'eggs'
  

.get（）方法返回所请求密钥的值，如果密钥缺失，则返回默认值。

或者，您可以为每个关键字创建一个带有空字符串的新dict，并使用 .update（） ：

  profile = dict.fromkeys（'id name first_name last_name link username gender locale'.split（），''）
 profile.update（result）
  

dict.fromkeys（）创建一个字典，其中所有要求的键设置为给定的默认值（''在上面的例子中），然后我们使用 .update（）从结果字典，替换任何已经在那里。

I currently have a Python 2.7 script which scrapes Facebook and captures some JSON data from each page. The JSON data contains personal information. A sample of the JSON data is below:-

{
   "id": "4",
   "name": "Mark Zuckerberg",
   "first_name": "Mark",
   "last_name": "Zuckerberg",
   "link": "http://www.facebook.com/zuck",
   "username": "zuck",
   "gender": "male",
   "locale": "en_US"
}

The JSON values can vary from page to page. The above example lists all the possibles but sometimes, a value such as 'username' may not exist and I may encounter JSON data such as:-

{
   "id": "6",
   "name": "Billy Smith",
   "first_name": "Billy",
   "last_name": "Smith",
   "gender": "male",
   "locale": "en_US"
}

With this data, I want to populate a database table. As such, my code is as below:-

results_json = simplejson.loads(scraperwiki.scrape(profile_url))
        for result in results_json:
            profile = dict()
            try:
                profile['id'] = int(results_json['id'])
            except:
                profile['id'] = ""
            try:
                profile['name'] = results_json['name']
            except:
                profile['name'] = ""
            try:
                profile['first_name'] = results_json['first_name']
            except:
                profile['first_name'] = ""
            try:
                profile['last_name'] = results_json['last_name']
            except:
                profile['last_name'] = ""
            try:
                profile['link'] = results_json['link']
            except:
                profile['link'] = ""
            try:
                profile['username'] = results_json['username']
            except:
                profile['username'] = ""
            try:
                profile['gender'] = results_json['gender']
            except:
                profile['gender'] = ""
            try:
                profile['locale'] = results_json['locale']
            except:
                profile['locale'] = ""

The reason I have so many try/excepts is to account for when the key value doesn't exist on the webpage. Nonetheless, this seems to be a really clumpsy and messy way to handle this issue.

If I remove these try / exception clauses, should my scraper encounter a missing key, it returns a KeyError such as "KeyError: 'username'" and my script stops running.

Any suggestions on a much smarter and improved way to handle these errors so that, should a missing key be encountered, the script continues.

I've tried creating a list of the JSON values and looked to iterate through them with an IF clause but I just can't figure it out.

解决方案

Use the .get() method instead:

>>> a = {'bar': 'eggs'}
>>> a['foo']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'foo'
>>> a.get('foo', 'default value')
'default value'
>>> a.get('bar', 'default value')
'eggs'

The .get() method returns the value for the requested key, or the default value if the key is missing.

Or you can create a new dict with empty strings for each key and use .update() on it:

profile = dict.fromkeys('id name first_name last_name link username gender locale'.split(), '')
profile.update(result)

dict.fromkeys() creates a dictionary with all keys you request set to a given default value ('' in the above example), then we use .update() to copy all keys and values from the result dictionary, replacing anything already there.

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