Roman numeral converter

romans.py

rmn = {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 roman(n):
    s = ''
    # This relies on behavior undefined before python 3.9. 
    for k in rmn.keys():
        while n - k >= 0:
            s = s + rmn[k]
            n = n - k
    return s

assert roman(5) == 'V'
assert roman(3549) == "MMMDXLIX"