How do I exchange keys with values in a dictionary?

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?

Solution 1:

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())

Solution 2:

new_dict = dict(zip(my_dict.values(), my_dict.keys()))

Solution 3:

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'}

Solution 4:

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'}

Solution 5:

You can make use of dict comprehensions:

Python 3

res = {v: k for k, v in a.items()}

Python 2

res = {v: k for k, v in a.iteritems()}

Edited: For Python 3, use a.items() instead of a.iteritems(). Discussions about the differences between them can be found in iteritems in Python on SO.