反向查找字典答案

【问题标题】：Reverse lookup Dictionary反向查找字典
【发布时间】：2021-01-21 06:33:47
【问题描述】：

我有以下字典，我想进行反向查找。

countries_dict = {'Andorra': ('Europe', 'Andorra la Vella'),
              'Afghanistan': ('Asia', 'Kabul'),
              'Antigua and Barbuda': ('North America', "St. John's"),
              'Albania': ('Europe', 'Tirana'),
              'Armenia': ('Asia', 'Yerevan'),
              'Angola': ('Africa', 'Luanda'),
              'Argentina': ('South America', 'Buenos Aires'),
              'Austria': ('Europe', 'Vienna'),
              'Australia': ('Oceania', 'Canberra'),
              'Azerbaijan': ('Asia', 'Baku'),
              'Barbados': ('North America', 'Bridgetown'),
              'Bangladesh': ('Asia', 'Dhaka'),
              'Belgium': ('Europe', 'Brussels')}

我的代码如下所示：

def create_continents_dict(dct = dict) -> dict:
    reversed_dictionary = {value[0] : key for (key, value) in dct.items()}
    return reversed_dictionary

print(create_continents_dict(countries_dict))

{'Europe': 'Belgium', 'Asia': 'Bangladesh', 'North America': 'Barbados', 'Africa': 'Angola', 'South America': 'Argentina', 'Oceania': 'Australia'}

但是，我只得到它查找为价值的最后一个国家/地区。我想要的是作为国家列表的价值。使用上面的代码，我应该期望拥有同一大陆的每个国家/地区，它应该在一个列表中，对吧？

{'Europe': ['Andorra', 'Albania', 'Austria', 'Belgium'] 'Asia': ['Afghanistan', 'Armenia', 'Azerbaijan', 'Bangladesh'], 'North America': ['Antigua and Barbuda', 'Barbados'], 'Africa': ['Angola'], 'South America': ['Argentina'], 'Oceania': ['Australia']}

【问题讨论】：

标签： python dictionary reverse-lookup

【解决方案1】：

您可以将dict.setdefault() 与默认列表（[]）一起使用，然后在其中附加国家/地区：

out = {}
for country, (continent, _) in countries_dict.items():
    out.setdefault(continent, []).append(country)

print(out)

打印：

{'Europe': ['Andorra', 'Albania', 'Austria', 'Belgium'], 
 'Asia': ['Afghanistan', 'Armenia', 'Azerbaijan', 'Bangladesh'], 
 'North America': ['Antigua and Barbuda', 'Barbados'], 
 'Africa': ['Angola'], 
 'South America': ['Argentina'], 
 'Oceania': ['Australia']}

【讨论】：

【解决方案2】：

你可以在这里使用collections.defaultdict

def reverse_dict(countries_dict):
    reversed_dictionary  = defaultdict(list)
    for key, value in countries_dict.items():
        reversed_dictionary[value[0]].append(key)
    return reversed_dictionary

>>> reverse_dict(countries_dict)
defaultdict(<class 'list'>,
            {'Europe': ['Andorra', 'Albania', 'Austria', 'Belgium'],
             'Asia': ['Afghanistan', 'Armenia', 'Azerbaijan', 'Bangladesh'],
             'North America': ['Antigua and Barbuda', 'Barbados'],
             'Africa': ['Angola'],
             'South America': ['Argentina'],
             'Oceania': ['Australia']})

【讨论】：

【解决方案3】：

试试

from collections import defaultdict 
countries_dict = {'Andorra': ('Europe', 'Andorra la Vella'),
              'Afghanistan': ('Asia', 'Kabul'),
              'Antigua and Barbuda': ('North America', "St. John's"),
              'Albania': ('Europe', 'Tirana'),
              'Armenia': ('Asia', 'Yerevan'),
              'Angola': ('Africa', 'Luanda'),
              'Argentina': ('South America', 'Buenos Aires'),
              'Austria': ('Europe', 'Vienna'),
              'Australia': ('Oceania', 'Canberra'),
              'Azerbaijan': ('Asia', 'Baku'),
              'Barbados': ('North America', 'Bridgetown'),
              'Bangladesh': ('Asia', 'Dhaka'),
              'Belgium': ('Europe', 'Brussels')}

d = defaultdict(list)
for k,v in countries_dict.items():
  d[v[0]].append(k)
print(d)

输出

defaultdict(<class 'list'>, {'Europe': ['Andorra', 'Albania', 'Austria', 'Belgium'], 'Asia': ['Afghanistan', 'Armenia', 'Azerbaijan', 'Bangladesh'], 'North America': ['Antigua and Barbuda', 'Barbados'], 'Africa': ['Angola'], 'South America': ['Argentina'], 'Oceania': ['Australia']})

【讨论】：

【解决方案4】：

def create_continents_dict(dct = dict) -> dict:
    reversed_dictionary = {}
    for key, value in dct.items():
        if value[0] not in reversed_dictionary.keys():
            reversed_dictionary[value[0]] = []
        reversed_dictionary[value[0]].append(key)
    return reversed_dictionary

输出：

print(create_continents_dict(countries_dict))
{'Europe': ['Andorra', 'Albania', 'Austria', 'Belgium'], 'Asia': ['Afghanistan', 'Armenia', 'Azerbaijan', 'Bangladesh'], 'North America': ['Antigua and Barbuda', 'Barbados'], 'Africa': ['Angola'], 'South America': ['Argentina'], 'Oceania': ['Australia']}

【讨论】：