Python对JSON值排序列表

Question

我有一个由JSON组成的文件，每个文件都有一行，并且希望按update_time reversed对文件进行排序。

示例JSON文件：

{ "page": { "url": "url1", "update_time": "1415387875"}, "other_key": {} }
{ "page": { "url": "url2", "update_time": "1415381963"}, "other_key": {} }
{ "page": { "url": "url3", "update_time": "1415384938"}, "other_key": {} }

想要输出：

{ "page": { "url": "url1", "update_time": "1415387875"}, "other_key": {} }
{ "page": { "url": "url3", "update_time": "1415384938"}, "other_key": {} }
{ "page": { "url": "url2", "update_time": "1415381963"}, "other_key": {} }

我的代码：

#!/bin/env python
#coding: utf8

import sys
import os
import json
import operator

#load json from file
lines = []
while True:
    line = sys.stdin.readline()
    if not line: break
    line = line.strip()
    json_obj = json.loads(line)
    lines.append(json_obj)

#sort json
lines = sorted(lines, key=lambda k: k['page']['update_time'], reverse=True)

#output result
for line in lines:
    print line

代码在示例JSON文件中运行良好，但如果JSON没有“更新时间”，则会引发KeyError异常。有没有其他的方法可以做到这一点？

Answer 1

编写一个使用try...except处理KeyError的函数，然后将其用作key参数，而不是lambda。

def extract_time(json):
    try:
        # Also convert to int since update_time will be string.  When comparing
        # strings, "10" is smaller than "2".
        return int(json['page']['update_time'])
    except KeyError:
        return 0

# lines.sort() is more efficient than lines = lines.sorted()
lines.sort(key=extract_time, reverse=True)

Answer 2

# sort json
lines = sorted(lines, key=lambda k: k['page'].get('update_time', 0), reverse=True)

Answer 3

可以将^{}与默认值一起使用：

lines = sorted(lines, key=lambda k: k['page'].get('update_time', 0), reverse=True)

示例：

>>> lines = [
...     {"page": {"url": "url1", "update_time": "1415387875"}, "other_key": {}},
...     {"page": {"url": "url2", "update_time": "1415381963"}, "other_key": {}},
...     {"page": {"url": "url3", "update_time": "1415384938"}, "other_key": {}},
...     {"page": {"url": "url4"}, "other_key": {}},
...     {"page": {"url": "url5"}, "other_key": {}}
... ]
>>> lines = sorted(lines, key=lambda k: k['page'].get('update_time', 0), reverse=True)
>>> for line in lines:
...     print line
... 
{'other_key': {}, 'page': {'url': 'url1', 'update_time': '1415387875'}}
{'other_key': {}, 'page': {'url': 'url3', 'update_time': '1415384938'}}
{'other_key': {}, 'page': {'url': 'url2', 'update_time': '1415381963'}}
{'other_key': {}, 'page': {'url': 'url4'}}
{'other_key': {}, 'page': {'url': 'url5'}}

尽管如此，我还是会遵循费迪南德建议的^{} principle方法——这样，当page键也丢失时，您也可以处理这种情况。让它失败并处理它要比检查各种各样的角落案例容易得多。