number_to_word.py

def num2word(num):
    ones = {1: "one", 2: "two", 3: "three", 4: "four",
            5: "five", 6: "six", 7: "seven", 8: "eight",
            9: "nine", 0: "zero", 10: "ten"}
    teens = {11: "eleven", 12: "twelve", 13: "thirteen",
             14: "fourteen", 15: "fifteen"}
    tens = {2: "twenty", 3: "thirty", 4: "forty",
            5: "fifty", 6: "sixty", 7: "seventy",
            8: "eighty", 9: "ninety"}
    lens = {3: "hundred", 4: "thousand", 6: "hundred", 7: "million",
            8: "million", 9: "million", 10: "billion"  # ,13:"trillion",11:"googol",
            }

    if num > 999999999:
        return "Number more than 1 billion"

    # Ones
    if num < 11:
        return ones[num]
    # Teens
    if num < 20:
        word = ones[num % 10] + "teen" if num > 15 else teens[num]
        return word
    # Tens
    if num > 19 and num < 100:
        word = tens[int(str(num)[0])]
        if str(num)[1] == "0":
            return word
        else:
            word = word + " " + ones[num % 10]
            return word

    # First digit for thousands,hundred-thousands.
    if len(str(num)) in lens and len(str(num)) != 3:
        word = ones[int(str(num)[0])] + " " + lens[len(str(num))]
    else:
        word = ""

    # Hundred to Million
    if num < 1000000:
        # First and Second digit for ten thousands.
        if len(str(num)) == 5:
            word = num2word(int(str(num)[0:2])) + " thousand"
        # How many hundred-thousand(s).
        if len(str(num)) == 6:
            word = word + " " + num2word(int(str(num)[1:3])) + \
                   " " + lens[len(str(num)) - 2]
        # How many hundred(s)?
        thousand_pt = len(str(num)) - 3
        word = word + " " + ones[int(str(num)[thousand_pt])] + \
               " " + lens[len(str(num)) - thousand_pt]
        # Last 2 digits.
        last2 = num2word(int(str(num)[-2:]))
        if last2 != "zero":
            word = word + " and " + last2
        word = word.replace(" zero hundred", "")
        return word.strip()

    left, right = '', ''
    # Less than 1 million.
    if num < 100000000:
        left = num2word(int(str(num)[:-6])) + " " + lens[len(str(num))]
        right = num2word(int(str(num)[-6:]))
    # From 1 million to 1 billion.
    if num > 100000000 and num < 1000000000:
        left = num2word(int(str(num)[:3])) + " " + lens[len(str(num))]
        right = num2word(int(str(num)[-6:]))
    if int(str(num)[-6:]) < 100:
        word = left + " and " + right
    else:
        word = left + " " + right
    word = word.replace(" zero hundred", "").replace(" zero thousand", " thousand")
    return word