The 1-liner (whitespace is unnecessary) extensible way to "increment" a string

gistfile1.py

INCREMENTED_MAP = {'1': ('2', False), '0': ('1', False), '3': ('4', False), '2': ('3', False), '5': ('6', False), '4': ('5', False), '7': ('8', False), '6': ('7', False), '9': ('0', True), '8': ('9', False)}
# Can extend INCREMENTED_MAP with 'a': ('b', False), ... 'z': ('a', True) to get lowercase alphabetics, etc

def incremented(number):
    return ''.join(
        reversed(
            reduce(
                lambda (so_far, carry_in), (old_digit, (new_digit, carry_out)):
                    (
                        so_far + (new_digit if carry_in else old_digit,),
                        carry_out
                    ),
                reversed(
                    zip(
                        number,
                        map(INCREMENTED_MAP.get, number))
                    ),
                    ((), True)
                )[0]
        )
    )

tests = (
    '123',
    '129',
    '199',
    '999',
)
for x in tests:
    print x, incremented(x)