>>> invert_shift("a")
    'A'
    >>> invert_shift("A")
    'a'

    >>> invert_shift("123 foo 456 BAR #!")
    '123 FOO 456 bar #!'

def invert_shift(chars):
    """
    >>> invert_shift("a")
    'A'
    >>> invert_shift("A")
    'a'

    >>> invert_shift("123 foo 456 BAR #!")
    '123 FOO 456 bar #!'
    """
    result = ""
    for char in chars:
        if char in string.ascii_lowercase:
#             log.critical("auto shift lowercase char %s to UPPERCASE", repr(char))
            char = char.upper()
        elif char in string.ascii_uppercase:
#             log.critical("auto shift UPPERCASE char %s to lowercase", repr(char))
            char = char.lower()
        result += char
    return result