Convert Roman numeral strings to integers #PyPool

numerals.py

"""Roman Numeral to Arabic Numbers.

Some doctests:

>>> convert_to_arabic('Nulla')
0
>>> convert_to_arabic('i')
1
>>> convert_to_arabic('v')
5
>>> convert_to_arabic('x')
10
>>> convert_to_arabic('l')
50
>>> convert_to_arabic('c')
100
>>> convert_to_arabic('d')
500
>>> convert_to_arabic('m')
1000
>>> convert_to_arabic('MMXiV')
2014
>>> c2a('MCMLIV')
1954
>>> c2a('MDCCCCLXXXXVIIII')
1999

Check for handling of expected errors:

>>> c2a('MMCIIV')
Traceback (most recent call last):
    ...
ValueError: MMCIIV non valet numerales Romani
>>> c2a('cow')
Traceback (most recent call last):
    ...
ValueError: O is not a roman numeral
>>> c2a('ivx')
Traceback (most recent call last):
    ...
ValueError: IVX non valet numerales Romani

"""

import sys

lookup_table = {
    'I': 1,
    'V': 5,
    'X': 10,
    'L': 50,
    'C': 100,
    'D': 500,
    'M': 1000
}

def _test():
    import doctest
    doctest.testmod(verbose=True)


def convert_to_arabic(roman):
    global lookup_table
    roman = roman.upper()
    number = 0
    inSubtract = False
    if roman == 'NULLA':
        return number
    # Check input only contains roman numerals
    for char in roman:
        if char not in lookup_table:
            raise ValueError("{char} is not a roman numeral".format(char=char))
    reversed_roman = roman[::-1]
    for i, char in enumerate(reversed_roman):
        if i == 0:
            number += lookup_table[char]
        elif lookup_table[char] < lookup_table[reversed_roman[i - 1]]:
            if inSubtract:
                # Must be 2 consecutive subtractions
                raise ValueError("{0} non valet numerales Romani".format(roman))
            number -= lookup_table[char]
            inSubtract = True
        elif lookup_table[char] <= lookup_table[reversed_roman[i - 1]] and inSubtract:
            raise ValueError("{0} non valet numerales Romani".format(roman))
        else:
            number += lookup_table[char]        
            inSubtract = False

    return number


# Alias function for less typing!
c2a = convert_to_arabic


if __name__ == "__main__" and '-t' in sys.argv:
    _test()
else:
    for roman in sys.argv[1:]:
        print convert_to_arabic(roman)