"""Convert to and from Roman numerals

This program is part of "Dive Into Python 3", a free Python book for
experienced programmers.  Visit http://diveintopython3.org/ for the
latest version.
"""
class OutOfRangeError(ValueError):
    pass

roman_numeral_map = (('M',  1000),
                     ('CM', 900),
                     ('D',  500),
                     ('CD', 400),
                     ('C',  100),
                     ('XC', 90),
                     ('L',  50),
                     ('XL', 40),
                     ('X',  10),
                     ('IX', 9),
                     ('V',  5),
                     ('IV', 4),
                     ('I',  1))

def to_roman(n):
    """convert integer to Roman numeral"""
    if n > 3999:
        raise OutOfRangeError("number out of range (must be less than 3999)")

    result = ""
    for numeral, integer in roman_numeral_map:
        while n >= integer:
            result += numeral
            n -= integer
    return result