roman1.py

import re

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))

to_roman_table = [ None ]
from_roman_table = {}

class OutOfRangeError(ValueError):
    pass
class NotIntegerError(ValueError):
    pass
class InvalidRomanNumeralError(ValueError):
    pass
class NotStringError(ValueError):
    pass

def to_roman(n):
    '''convert integer to Roman numeral'''
    if n > 4999 or n <=0:
        raise OutOfRangeError('number out of range (must be 1..4999')
    if int(n) != n :
        raise NotIntegerError('non-integers cannot be converted')
    return to_roman_table[n]

def from_roman(s):
    '''convert Roman numeral to integer'''
    if not isinstance(s,str):
        raise NotStringError('non-strings cannot be converted')
    if not s:
        raise InvalidRomanNumeralError('Input cannot be blank')
    if s not in from_roman_table:    
        raise InvalidRomanNumeralError('Invalid Roman numeral: {}'.format(s))
    return from_roman_table[s]

def build_lookup_tables():
    def to_roman(n):
        result = ''
        for numeral, integer in roman_numeral_map:
            if n >= integer:
                result = numeral
                n -= integer
                break
        if n > 0:
            result += to_roman_table[n]
        return result
            
    for integer in range(1, 5000):
        roman_numeral = to_roman(integer)
        to_roman_table.append(roman_numeral)
        from_roman_table[roman_numeral] = integer
        
build_lookup_tables()