Python创建列表的字典 [英] Python creating a dictionary of lists

问题描述

我想创建一个值为列表的字典。例如：

I want to create a dictionary whose values are lists. For example:

{
  1: ['1'],
  2: ['1','2'],
  3: ['2']
}

如果我这样做：

d = dict()
a = ['1', '2']
for i in a:
    for j in range(int(i), int(i) + 2): 
        d[j].append(i)

我收到一个KeyError，因为d [...]不是列表。在这种情况下，我可以在分配a之后添加以下代码来初始化字典。

I get a KeyError, because d[...] isn't a list. In this case, I can add the following code after the assignment of a to initialize the dictionary.

for x in range(1, 4):
    d[x] = list()

有更好的方法吗？让我说我不知道​​我将需要的密钥，直到我在循环的第二个中。例如：

Is there a better way to do this? Lets say I don't know the keys I am going to need until I am in the second for loop. For example:

class relation:
    scope_list = list()
...
d = dict()
for relation in relation_list:
    for scope_item in relation.scope_list:
        d[scope_item].append(relation)

然后，另一种替代方法是替换

An alternative would then be replacing

d[scope_item].append(relation)

与

if d.has_key(scope_item):
    d[scope_item].append(relation)
else:
    d[scope_item] = [relation,]

最好的处理方法是什么？理想情况下，追加会正常工作。有没有办法表达我想要一个空列表的字典，即使我第一次创建列表时我不知道每个键？

What is the best way to handle this? Ideally, appending would "just work". Is there some way to express that I want a dictionary of empty lists, even if I don't know every key when I first create the list?

推荐答案

您可以使用 defaultdict ：

>>> from collections import defaultdict
>>> d = defaultdict(list)
>>> for i in a:
...   for j in range(int(i), int(i) + 2):
...     d[j].append(i)
...
>>> d
defaultdict(<type 'list'>, {1: ['1'], 2: ['1', '2'], 3: ['2']})
>>> d.items()
[(1, ['1']), (2, ['1', '2']), (3, ['2'])]

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