Python - Number to words

number.py

"""
    This script returns a string of a number in letters (**in french**) from a positive integer.
    
    $ python3 number.py 1254
    mille deux-cent cinquante-quatre
    
    It works for numbers from 0 to 999 999 999 999 (1 trillion minus 1).
"""

import sys

SINGLE_NUMBERS = {
    0: "zéro",
    1: "un",
    2: "deux",
    3: "trois",
    4: "quatre",
    5: "cinq",
    6: "six",
    7: "sept",
    8: "huit",
    9: "neuf",
    10: "dix",
    11: "onze",
    12: "douze",
    13: "treize",
    14: "quatorze",
    15: "quinze",
    16: "seize"
}

NUMBER_NAMES = {
    "units": {
        1: "et-un",
        2: "deux",
        3: "trois",
        4: "quatre",
        5: "cinq",
        6: "six",
        7: "sept",
        8: "huit",
        9: "neuf"
    },
    "tens": {
        1: "dix",
        2: "vingt",
        3: "trente",
        4: "quarante",
        5: "cinquante",
        6: "soixante",
        7: "septente",
        8: "octante",
        9: "nonante"
    }
}

def render_tens_units(tens, units):
    if units == 0:
        return NUMBER_NAMES["tens"][tens]
    return NUMBER_NAMES["tens"][tens] + '-' + NUMBER_NAMES["units"][units]

def render(n):
    if n < 0 or type(n) is not int:
        raise ValueError("'n' must be a positive integer.")
    if n < 17:
        return SINGLE_NUMBERS.get(n)
    if n < 100:
        tens, units = divmod(n, 10)
        return render_tens_units(tens, units)
    if n < 1000:
        hundreds, rest = divmod(n, 100)
        tens, units = divmod(rest, 10)
        if hundreds == 1:
            h = "cent"
        else:
            h = SINGLE_NUMBERS.get(hundreds) + "-cent"
        return h + ' ' + render_tens_units(tens, units)
    if n < 10000:
        thousands, rest = divmod(n, 1000)
        if thousands == 1:
            t = "mille"
        else:
            t = SINGLE_NUMBERS.get(thousands) + "-mille"
        return t + ' ' + render(rest)
    if n < 1000000:
        thousands, rest = divmod(n, 1000)
        return render(thousands) + "-mille " + render(rest)
    if n < 1000000000:
        millions, rest = divmod(n, 1000000)
        if rest == 0:
            return render(millions) + "-millions"
        return render(millions) + "-millions " + render(rest)
    if n < 1000000000000:
        billions, rest = divmod(n, 1000000000)
        if rest == 0:
            return render(billions) + "-milliards"
        return render(billions) + "-milliards " + render(rest)
    return

if __name__ == '__main__':
    print(render(int(sys.argv[1])))