如何使用映射或过滤器而不是列表推导为特定值过滤嵌套字典(Python方式)? [英] How to filter a nested dictionary (pythonic way) for a specific value using map or filter instead of list comprehensions?
问题描述
我有一个嵌套的字典.
>>> foo = {'m': {'a': 10}, 'n': {'a': 20}}
>>>
我想根据'a'的值过滤特定的值.
I'd like to filter specific values, based on the values of 'a'.
我可以为此目的使用列表推导.
I can use list comprehensions for the purpose.
>>> [foo[n] for n in foo if foo[n]['a'] == 10]
[{'a': 10}]
>>>
仅使用list即可给我foo中的元素(而不是元素的值)-符合预期:
Using list alone gives me the elements from foo (and not the values of the elements) - as expected:
>>> list(filter(lambda x: foo[x] if foo[x]['a']==10 else None,foo))
['m']
>>>
使用地图会返回不需要的无"值:
Using map returns me unwanted 'None' values:
>>> list(map(lambda x: foo[x] if foo[x]['a']==10 else None,foo))
[{'a': 10}, None]
>>>
结合这两个，我可以获取所需的值.但是我想foo被迭代了两次-一次用于过滤器和映射.列表理解解决方案只需要我重复一次即可.
Combining these two, I can fetch the desired value. But I guess foo is iterated twice - once each for filter and map. The list comprehension solution needs me to iterate just once.
>>> list(map(lambda t: foo[t], filter(lambda x: foo[x] if foo[x]['a']==10 else None,foo)))
[{'a': 10}]
>>>
这是仅使用过滤器的另一种方法.这给了我想要的值，但是我不确定对字典的值进行迭代是否是一种好方法/pythonic方法:
Here's another approach using just filter. This gives me the desired values but I'm not sure if iterating over values of a dictionary is a good/pythonic approach:
>>> list(filter(lambda x: x if x['a'] == 10 else None, foo.values()))
[{'a': 10}]
>>>
我想知道是否:
- 在这种情况下，pythonic/推荐的方法是什么?
- 如果最后一个在字典值上使用过滤器的示例是否可以接受?
致谢
Sharad
推荐答案
列表理解可以做到这一点.
A list comprehension can do this beautifully.
>>> foo = {'m': {'a': 10}, 'n': {'a': 20}}
>>> [v for v in foo.values() if 10 in v.values()]
[{'a': 10}]
如果您与字典中的已知键匹配，则不需要for循环或列表理解.
You don't need the for loop or the list comprehension if you are matching against a known key in the dictionary.
In [15]: if 10 in foo['m'].values():
...: result = [foo['m']]
...:
In [16]: result
Out[16]: [{'a': 10}]
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