python - 用字典中的值交换键的改进？

Question

我读过两个关于用字典中的值交换键的问题：Python: Best Way to Exchange Keys with Values in a Dictionary? ; 如何交换字典中的值的键？.

推广的最佳方法是：

dict((v,k) for k,v in d.items())

但是，如果像这样的字典：

d = {"one":1, "two":2, "a":1}

mothed 得到交换的字典：

{1: 'one', 2: 'two'}

项目 (1:'a') 丢失。那么我怎样才能得到这样的字典 {1: ('one', 'a'), 2: 'two'}？谢谢。

Answer 1

这是一种方法：

In [101]: d = {"one":1, "two":2, "a":1}

In [102]: answer = collections.defaultdict(list)

In [103]: for k,v in d.iteritems():
   .....:     answer[v].append(k)

   .....:     

In [104]: answer
Out[104]: defaultdict(<type 'list'>, {1: ['a', 'one'], 2: ['two']})

In [105]: dict(answer)
Out[105]: {1: ['a', 'one'], 2: ['two']}

In [106]: dict((k, tuple(v)) for k,v in answer.iteritems())
Out[106]: {1: ('a', 'one'), 2: ('two',)}

如果你真的想要一个单线（我强烈建议不要这样做 - 它非常低效）：

In [110]: d
Out[110]: {'a': 1, 'one': 1, 'two': 2}

In [111]: dict((v, tuple([key for key in d if d[key]==v])) for k,v in d.iteritems())
Out[111]: {1: ('a', 'one'), 2: ('two',)}

希望这可以帮助

Answer 2

你可以使用dict.setdefault()：

In [6]: dd={}

In [7]: for x,y in d.items():
   ...:     dd.setdefault(y,[]).append(x)
   ...:     

In [8]: dd
Out[8]: {1: ['a', 'one'], 2: ['two']}

或者您也可以使用defaultdict()：

In [1]: from collections import defaultdict

In [2]: d = {"one":1, "two":2, "a":1}

In [3]: dd=defaultdict(list)

In [4]: for x,y in d.items():
   ...:     dd[y].append(x)
   ...:     

In [5]: dd
Out[5]: defaultdict(<type 'list'>, {1: ['a', 'one'], 2: ['two']})

Answer 3

d = {"one":1, "two":2, "a":1}
a = {}
for k,v in d.iteritems():
    a.setdefault(v, []).append(k)

如果它必须是单线：

import itertools as it
import operator as op

a = {k:tuple(b[0] for b in v) for k,v in it.groupby(sorted(d.items(), key=op.itemgetter(1)), key=op.itemgetter(1))}

Answer 4

我不确定一个班轮，但这有效：

old = {"one": 1, "two": 2, "a": 1}
new = {}

for key, val in old.iteritems():
    new[val] = new.get(val, []) + [key]