[英]Python 2.7: Appending to a list value of a dictionary key
I have the following data: 我有以下数据:
data = [(1, 2), (1, 3), (1, 4), (2, 1), (2, 2), (2, 3)]
and I want to create a dictionary which contains key-list value, how can I do this with a dictionary comprehension? 我想创建一个包含键列表值的字典,我怎样才能用字典理解呢?
ie: 即:
{1: [2,3,4]
2: [1,2,3]
}
I have tried the following but the list gets overwritten on every iteration. 我尝试了以下内容,但列表在每次迭代时都会被覆盖。
{x: [y] for x,y in data}
You can use this dict-comprehension: 你可以使用这个词典理解:
d = {x: [v for u,v in data if u == x] for x,y in data}
Note, however, that this is pretty inefficient, as it will loop the entire list n+1
times! 但请注意,这是非常低效的,因为它将循环整个列表
n+1
次!
Better use just a plain-old for
loop: 更好地使用一个普通的
for
循环:
d = {}
for x,y in data:
d.setdefault(x, []).append(y)
Alternatively, you could also use itertools.groupy
(as discovered by yourself): 或者,您也可以使用
itertools.groupy
(由您自己发现):
groups = itertools.groupby(sorted(data), key=lambda x: x[0])
d = {k: [g[1] for g in group] for k, group in groups}
In all cases, d
ends up being {1: [2, 3, 4], 2: [1, 2, 3]}
在所有情况下,
d
最终为{1: [2, 3, 4], 2: [1, 2, 3]}
You could use defaultdict
from collections module. 您可以使用collections模块中的
defaultdict
。
>>> from collections import defaultdict
>>> data = [(1, 2), (1, 3), (1, 4), (2, 1), (2, 2), (2, 3)]
>>> m = defaultdict(list)
>>> for i,j in data:
m[i].append(j)
>>> m
defaultdict(<class 'list'>, {1: [2, 3, 4], 2: [1, 2, 3]})
>>> dict(m)
{1: [2, 3, 4], 2: [1, 2, 3]}
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