How do I exchange keys with values in a dictionary?

In [1]: my_dict = {'x':1, 'y':2, 'z':3}

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

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

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

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'}

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())

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