roman_first.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))
roman_numeral_pattern = re.compile('''
    ^                   # beginning of string
    M{0,4}              # thousands - 0 to 3 Ms
    (CM|CD|D?C{0,3})    # hundreds - 900 (CM), 400 (CD), 0-300 (0 to 3 Cs),
                        #            or 500-800 (D, followed by 0 to 3 Cs)
    (XC|XL|L?X{0,3})    # tens - 90 (XC), 40 (XL), 0-30 (0 to 3 Xs),
                        #        or 50-80 (L, followed by 0 to 3 Xs)
    (IX|IV|V?I{0,3})    # ones - 9 (IX), 4 (IV), 0-3 (0 to 3 Is),
                        #        or 5-8 (V, followed by 0 to 3 Is)
    $                   # end of string
    ''', re.VERBOSE)

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 not isinstance(n,int):
        raise NotIntegerError('non-integers cannot 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'''
    if not isinstance(s,str):
        raise NotStringError('non-strings cannot be converted')
    if not s:
        raise InvalidRomanNumeralError('Input cannot be blank')
    if not roman_numeral_pattern.search(s):
        raise InvalidRomanNumeralError('Invalid Roman numeral: {}'.format(s))
    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