dive-into-python3/roman5.py

"""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
class NotIntegerError(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 not (0 < n < 4000):
        raise OutOfRangeError("number out of range (must be 0..3999)")
    if not isinstance(n, int):
        raise NotIntegerError("non-integers can not be converted")

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

def from_roman(s):
    """convert Roman numeral to integer"""
    result = 0
    index = 0
    for numeral, integer in roman_numeral_map:
        while s[index : index + len(numeral)] == numeral:
            result += integer
            index += len(numeral)
    return result