How do I exchange keys with values in a dictionary?

user_103532 0
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I receive a dictionary as input, and would like to to return a dictionary whose keys will be the input's values and whose value will be the corresponding input keys. Values are unique.

For example, say my input is:

a = dict()
a['one']=1
a['two']=2

I would like my output to be:

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

To clarify I would like my result to be the equivalent of the following:

res = dict()
res[1] = 'one'
res[2] = 'two'

Any neat Pythonic way to achieve this?

reverse mapping dictionary python
19 Answers

Python 2:

res = dict((v,k) for k,v in a.iteritems())

Python 3 (thanks to @erik):

res = dict((v,k) for k,v in a.items())
user_426100 
new_dict = dict(zip(my_dict.values(), my_dict.keys()))
Javier_116490

From Python 2.7 on, including 3.0+, there's an arguably shorter, more readable version:

>>> my_dict = {'x':1, 'y':2, 'z':3}
>>> {v: k for k, v in my_dict.items()}
{1: 'x', 2: 'y', 3: 'z'}
user_128550 
In [1]: my_dict = {'x':1, 'y':2, 'z':3}

Python 3

In [2]: dict((value, key) for key, value in my_dict.items())
Out[2]: {1: 'x', 2: 'y', 3: 'z'}

Python 2

In [2]: dict((value, key) for key, value in my_dict.iteritems())
Out[2]: {1: 'x', 2: 'y', 3: 'z'}
user_1228510

You could try:

Python 3

d={'one':1,'two':2}
d2=dict((value,key) for key,value in d.items())
d2
  {'two': 2, 'one': 1}

Python 2

d={'one':1,'two':2}
d2=dict((value,key) for key,value in d.iteritems())
d2
  {'two': 2, 'one': 1}

Beware that you cannot 'reverse' a dictionary if

  1. More than one key shares the same value. For example {'one':1,'two':1}. The new dictionary can only have one item with key 1.
  2. One or more of the values is unhashable. For example {'one':[1]}. [1] is a valid value but not a valid key.

See this thread on the python mailing list for a discussion on the subject.

user_1139620

res = dict(zip(a.values(), a.keys()))

pkit_41660 
new_dict = dict( (my_dict[k], k) for k in my_dict)

or even better, but only works in Python 3:

new_dict = { my_dict[k]: k for k in my_dict}
user_1158660

Can be done easily with dictionary comprehension:

{d[i]:i for i in d}
user_100844430

Hanan's answer is the correct one as it covers more general case (the other answers are kind of misleading for someone unaware of the duplicate situation). An improvement to Hanan's answer is using setdefault:

mydict = {1:a, 2:a, 3:b}   
result = {}
for i in mydict:  
   result.setdefault(mydict[i],[]).append(i)
print(result)
>>> result = {a:[1,2], b:[3]}
user_78242040

I find this version the most comprehensive one:

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

swapped_a = {value : key for key, value in a.items()}

print(swapped_a)

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

user_23725000

An alternative that is not quite as readable (in my opinion) as some of the other answers:

new_dict = dict(zip(*list(zip(*old_dict.items()))[::-1]))

where list(zip(*old_dict.items()))[::-1] gives a list of 2 tuples, old_dict's values and keys, respectively.

user_123526910
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