检查字典键是否存在并具有值的有效方法 [英] Efficient way to check if dictionary key exists and has value

问题描述

我们假设有一本字典如下:

Let's say there's a dictionary that looks like this:

d = {"key1": "value1", "key2": {"key3": "value3"}}

字典可能包含或可能不包含 key2 ，而且 key2 可能为空.因此，为了获得 value3 ，我需要检查 key2 及其值是否存在非空值， key3 也是如此.

The dictionary may or may not contain key2, also key2 might be empty. So in order To get value3, I would need to check if key2 and its value exist with non-null values, same applies for key3.

现在显而易见的愚蠢解决方案是这样的:

Now the obvious stupid solution would be like this:

if 'key2' in d:
    if d['key2']:
        if 'key3' in d['key2']:
            value = d['key2']['key3']

现在，我想知道是否有一个更简单的解决方案，所以我不必连续写3个 if .

Now, I wonder if there's a simpler solution so I don't have to write 3 ifs in a row.

推荐答案

一种方法是预见故障并予以捕获:

One approach would be to expect the failure and catch it:

try:
    value = d['key2']['key3']
except (KeyError, TypeError):
    pass

(不要将变量命名为类型的名称，这是一种不好的做法，我已将其重命名为 d )

(don't call your variable the name of the type, it's a bad practice, I've renamed it d)

KeyError 捕获丢失的键， TypeError 捕获尝试索引不是 dict 的内容的

The KeyError catches a missing key, the TypeError catches trying to index something that's not a dict.

如果您希望这种失败非常普遍，那么这可能并不理想，因为 try ..除了块有一些开销.

If you expect this failure to be very common, this may not be ideal, as there's a bit of overhead for a try .. except block.

在这种情况下，尽管我将其写为:

In that case you're stuck with what you have, although I'd write it as:

if 'key2' in d and d['key2'] and 'key3' in d['key2']:
    value = d['key2']['key3']

或者可能更清楚一点:

if 'key2' in d and isinstance(d['key2'], dict) and 'key3' in d['key2']:
    value = d['key2']['key3']

如果您要在 else 部分的 value 中分配其他内容(例如 None )，则还可以考虑:

If you're about to assign something else to value in the else part (like None), you could also consider:

value = d['key2']['key3'] if 'key2' in d and d['key2'] and 'key3' in d['key2'] else None
    

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