Since rot_13 is a transcoder, not an encoder, the error message is correct, as is the fix for the function. However, since neither the module encodings.rot_13 nor the rot13 function in the module are documented, (not even in 2.7), I wonder if the function and the if __name__; clause are ancient holdovers that should be removed. In other words, I am not sure if Berker's test is currently a supported usage. I remember that there was some discussion on pydev about how much to add back but not the details.