从Python字典中读取是否可能不存在密钥 [英] Reading from Python dict if key might not be present

问题描述

我对Python和解析数据非常陌生.

I am very new to Python and parsing data.

我可以将外部JSON提要放入Python字典中并在字典上进行迭代.

I can pull an external JSON feed into a Python dictionary and iterate over the dictionary.

for r in results:
     print r['key_name']

当我浏览返回的结果时，如果键没有值(记录中的值可能并不总是存在)，则会出现错误.如果我打印结果，则显示为

As I walk through the results returned, I am getting an error when a key does not have a value (a value may not always exist for a record). If I print the results, it shows as

'key_name': None, 'next_key':.................

我的代码因错误而中断.如何控制没有值的键?

My code breaks on the error. How can I control for a key not having a value?

任何帮助将不胜感激！

布鲁克

推荐答案

适用的首选方法:

for r in results:
     print r.get('key_name')

如果key_name不是字典中的键，则仅打印None.您还可以使用其他默认值，只需将其作为第二个参数传递即可:

this will simply print None if key_name is not a key in the dictionary. You can also have a different default value, just pass it as the second argument:

for r in results:
     print r.get('key_name', 'Missing: key_name')

如果您想做一些与使用默认值不同的事情(例如，在没有键的情况下完全跳过打印)，那么您需要更多的结构，即:

If you want to do something different than using a default value (say, skip the printing completely when the key is absent), then you need a bit more structure, i.e., either:

for r in results:
    if 'key_name' in r:
        print r['key_name']

或

for r in results:
    try: print r['key_name']
    except KeyError: pass

第二个可以更快(如果它相当稀有，而缺少一个键)，但是对于许多人来说，第一个看起来更自然.

the second one can be faster (if it's reasonably rare than a key is missing), but the first one appears to be more natural for many people.

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