Improvement for swapping keys with values in a dictionay?

Question

I've read two questions about exchange keys with values in a dictionary: Python: Best Way to Exchange Keys with Values in a Dictionary?; How to swap keys for values in a dictionary?.

The best mothod promoted is:

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

However, if a dictionary like this:

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

the mothed gets the swapped dictionary:

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

The item (1:'a') gets lost. So how can I get a dictionary like this {1: ('one', 'a'), 2: 'two'}? Thanks.

score 4 · Accepted Answer · answered Nov 20 '12 at 07:33

Here's one way of doing it:

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',)}

If you really want a one-liner (I highly recommend against this - it's very inefficient):

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',)}

Hope this helps

@AshwiniChaudhary: Many thanks :). Coincidentally, I just got that badge earlier today. — inspectorG4dget, Nov 20 '12 at 07:43

score 4 · Answer 2 · edited Nov 20 '12 at 07:50

4

you can use 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']}

or you can also use 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']})

edited Nov 20 '12 at 07:50

Rohit Jain

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answered Nov 20 '12 at 07:33

Ashwini Chaudhary

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Did we both just post this at the exact same time? (mind=blown) – inspectorG4dget Nov 20 '12 at 07:35
@inspectorG4dget NO, you won by 8 seconds. :) – Ashwini Chaudhary Nov 20 '12 at 07:36
@AshwiniChaudhary.. Hey, can you tell me how to use `iPython`? Currently I use IDLE interpreter. I have downloaded the module - iPython, but unable to get how to use it. – Rohit Jain Nov 20 '12 at 07:38
@RohitJain I am on ubuntu, never used it in windows. just do :`sudo apt-get install ipython`, if you're on ubuntu. – Ashwini Chaudhary Nov 20 '12 at 07:40
@AshwiniChaudhary.. I'm on Windows :-\ – Rohit Jain Nov 20 '12 at 07:41
@RohitJain: Get the windows install instructions [here](http://ipython.org/ipython-doc/stable/install/install.html) – inspectorG4dget Nov 20 '12 at 07:42
@inspectorG4dget.. Can you join me on http://chat.stackoverflow.com/rooms/6/python – Rohit Jain Nov 20 '12 at 07:45
@RohitJain as Ipython runs in cmd in windows, you might have to download a better terminal application coz copying a text in cmd in pain in the ass. – Ashwini Chaudhary Nov 20 '12 at 07:46
@AshwiniChaudhary.. Yeah you are right. On my laptop, I installed it, and got it work on cmd. Thought there might be some better GUI, that I am unable to get. – Rohit Jain Nov 20 '12 at 07:49
@RohitJain IPython website have suggested this for windows: http://sourceforge.net/projects/console/ – Ashwini Chaudhary Nov 20 '12 at 07:51
@RohitJain: Another useful Python shell that works well on Windows is [DreamPie](http://www.dreampie.org/). You might want to check it out. – BrenBarn Nov 20 '12 at 07:55
@BrenBarn.. Thanks Bren. Will have to wait till I reach home to get that. Download site blocked in office :( – Rohit Jain Nov 20 '12 at 07:58
+1 for `setdefault()` I prefer it to `defaultdict` in most cases, just because it's part of the built-in API – acjay Nov 20 '12 at 08:16
1

@acjohnson55 `setdefault()` based solution is also faster than `defaultdict()`. – Ashwini Chaudhary Nov 20 '12 at 08:18

score 2 · Answer 3 · answered Nov 20 '12 at 07:35

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

If it has to be a one-liner:

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))}

score 0 · Answer 4 · answered Nov 20 '12 at 07:33

0

I'm not sure of a one liner, but this works:

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

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

answered Nov 20 '12 at 07:33

Tim

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Improvement for swapping keys with values in a dictionay?

4 Answers4