Use this code (trivially modified) from the accepted answer at Python reverse / invert a mapping:
dict((v,k) for k, v in my_dict.iteritems())
Note that this assumes that the values in the original dictionary are unique. Otherwise you'd end up with duplicate keys in the resulting dictionary, and that is not allowed.
And, as @wim points out, it also assumes the values are hashable. See the Python glossary if you're not sure what is and isn't hashable.
