roman.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

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

def to_roman(value):
    """Convert integer value to roman numeral string.

    Keyword arguments:
    value -- Converted value.

    Returns: string
    """

    value = int(value)
    if value <= 0:
        raise ValueError("Value too small (<= 0)")

    steps = sorted(ROMAN_NUMBERS.keys())
    output = ""

    while value > 0:
        step = steps[-1]
        if step <= value: # Substract and convert
            value -= step
            output += ROMAN_NUMBERS[step]
        else: # Move to lesser one
            steps.pop()

    return output

def from_roman(value):
    """Convert string of roman numerals to number.

    Keyword arguments:
    value -- String representing roman numeral.

    Returns: integer, or None if the input is empty string
    """

    # Backward conversion
    SYMBOLS = {symbol: value for value, symbol in ROMAN_NUMBERS.items()}
    SYM_MAX = max([len(symbol) for symbol in SYMBOLS.keys()])
    output = 0

    remains = value.strip()
    while len(remains) > 0:
        if remains.startswith(tuple(SYMBOLS.keys())):
            for l in range(SYM_MAX, 0, -1): # find the prefix, start from longest
                if remains[:l] in SYMBOLS: 
                    output += SYMBOLS[remains[:l]]
                    remains = remains[l:]
                    break
        else:
            raise ValueError("Invalid roman numeral")
    
    if output == 0:
        return None
    else:
        return output

if __name__ == '__main__':
    from sys import stdin
    from argparse import ArgumentParser, SUPPRESS

    def words(istream=stdin):
        """Yields words from input stream."""
        for line in istream:
            for word in line.split(): yield word

    parser = ArgumentParser(description="Converts numbers to/from roman numerals.")
    conversions = parser.add_mutually_exclusive_group(required=False)
    conversions.add_argument('-t', '--to-roman', dest="number", nargs='*', type=int, default=SUPPRESS,
            help="Convert integer to roman number"
            )
    conversions.add_argument('-f', '--from-roman', dest="roman", nargs='*', type=str, default=SUPPRESS,
            help="Convert roman literal to integer"
            )

    arguments = vars(parser.parse_args())

    # Source - an iterable which produces words to be converted
    # Converter - actual converting function
    if "number" in arguments: # To roman
        (source, converter) = (arguments["number"], to_roman)
    elif "roman" in arguments: # To integer
        (source, converter) = (arguments["roman"], from_roman)
    else: # Default - try to roman
        (source, converter) = (None, to_roman)

    # Detect "empty" input and switch to stdin
    if source is None or len(source) == 0: source = words(stdin)

    try:
        for value in source: print(converter(value))
    except ValueError as err:
        raise SystemExit("Error: " + str(err))