Short rot13 function - Python

I am searching for a short and cool rot13 function in Python ;-) I've written this function:

def rot13(s):
    chars = "abcdefghijklmnopqrstuvwxyz"
    trans = chars[13:]+chars[:13]
    rot_char = lambda c: trans[chars.find(c)] if chars.find(c)>-1 else c
    return ''.join( rot_char(c) for c in s ) 

Can anyone make it better? E.g supporting uppercase characters.