Convert numbers to their string representation.

num2str.py

import re

units = {
    0: 'Zero', 1: 'One', 2: 'Two', 3: 'Three', 4: 'Four', 5: 'Five', 6: 'Six', 7: 'Seven', 8: 'Eight', 9: 'Nine'
}
tens = {
    10: 'Ten', 11: 'Eleven', 12: 'Twelve', 13: 'Thirteen', 14: 'Fourteen', 15: 'Fifteen', 16: 'Sixteen',
    17: 'Seventeen', 18: 'Eighteen', 19: 'Nineteen', 20: 'Twenty', 30: 'Thirty', 40: 'Forty', 50: 'Fifty',
    60: 'Sixty', 70: 'Seventy', 80: 'Eighty', 90: 'Ninety'
}
groups = {
    0: '', 1: 'Thousand', 2: 'Million', 3: 'Billion', 4: 'Trillion', 5: 'Quadrillion', 6: 'Quintrillion',
    7: 'Sextillion', 8: 'Septillion', 9: 'Octillion', 10: 'Nonillion', 11: 'Decillion'
}


def convertDigit(d):
    return units[d]


def convertTwoDigits(d1, d2):
    if d2 == 0:
        return tens[d1 * 10]
    elif d1 == 1:
        return tens[d1 * 10 + d2]
    else:
        return tens[d1 * 10] + ' ' + convertDigit(d2)


def convertThreeDigits(d1, d2, d3):
    if d1 == 0 and d2 == 0 and d3 == 0:
        return ''

    output = ''
    if d1 != 0:
        output = convertDigit(d1) + ' Hundred'

    if d2 != 0:
        output = output + ' ' + convertTwoDigits(d2, d3)
    elif d3 != 0:
        output = output + ' ' + convertDigit(d3)

    return output


def convertGroup(index):
    return groups[index]


def chunks(s, n):
    for start in range(0, len(s), n):
        yield s[start:start + n]


def num2str(number):
    output = ''
    num = str(number)

    if num[0] == '-':
        output = 'Negative '
        num = num[1:]
    elif num[0] == '+':
        num = num[1:]

    if num[0] == '0':
        output = output + 'Zero'
    else:
        num = num.zfill(36)
        groups = chunks(num, 3)

        z = 0
        for group in groups:
            temp = convertThreeDigits(int(group[0]), int(group[1]), int(group[2]))
            if temp != '':
                output = output + temp + ' ' + convertGroup(11 - z) + ' '
            z = z + 1

    output = output.strip()
    output = re.sub(r'[ ]+', r' ', output)
    return output


if __name__ == '__main__':
    assert 'One' == num2str(1)
    assert 'Twelve' == num2str(12)
    assert 'Seventy Two' == num2str(72)
    assert 'Three Hundred' == num2str(300)
    assert 'One Thousand Two Hundred' == num2str(1200)
    assert 'Twelve Thousand Three Hundred Four' == num2str(12304)
    assert 'Six Million' == num2str(6000000)
    assert 'Six Million Four Hundred Thousand Five' == num2str(6400005)
    assert 'One Hundred Twenty Three Billion Four Hundred Fifty Six Million Seven Hundred Eighty Nine Thousand Twelve' == num2str(123456789012)
    assert 'Four Decillion' == num2str(4000000000000000000000000000000000)

    print 'All tests passed'