Python: Best Way to Exchange Keys with Values in a Dictionary?

Question

Hello,

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 Pythonian way to achieve this?

Thanks

Answer 1

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

Answer 2

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

Answer 3

You could try:

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.