Convert a number to its English description

number_to_words.py

ZERO = "zero"
DIGITS = ["one", "two", "three", "four", "five", "six", "seven", "eight", "nine"]
TEENS = ["ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "ninteen"]
TENS = ["twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"]
GROUPS = ["hundred", "thousand", "million", "billion", "trillion"]
 
def number_to_words(n):
    """
    Converts a number, n, into its English description. 
    
    n: 0 <= n <= 999999999999999
    
    This works by breaking the number up into groups of three digits from right 
    to left. Each group of three digits is converted into its english description, 
    then the appropriate suffix is appended. For example:
 
    => 1311453
    => 1 | 311 | 453
    => one | three hundred eleven | four hundred fifty three
    => one + million | three hundred eleven + thousand | four hundred fifty three
    => one million, three hundred eleven thousand, four hundred fifty three
 
    ex. 
    >>> print numbers_to_words(3451432)
    >>> three million, four hundred fifty one thousand, four hundred thirty two
    """
    if n == 0:
        return ZERO
    
    # keep track of which group of three we are on
    group_count = 1
    word = ""
    
    while n > 0:
        # get the next three digits of the number
        group = n % 1000
        
        # remove those three digits from the number
        n = n // 1000
        
        # now we can process these three digits
        digit_count = 1
        group_word = ""
            
        while group > 0:
            # get the current digit
            digit = group % 10
            # remove this digit from the number
            group = group // 10
            # peek at the next digit 
            next_digit = group % 10
            # note which digit we are at in this group of three
            group_position = digit_count % 3
           
            if group_position == 1 and next_digit == 1:
                # we are on the first digit, but the next digit is a one, 
                # so we can go ahead and process the first two digits at once
                group_word = TEENS[digit]
                # make sure we process the third digit next time
                group = group // 10
                digit_count += 1
            elif group_position == 1 and digit > 0:
                # first digit (ones)
                group_word = DIGITS[digit - 1]
            elif group_position == 2 and digit > 0:
                # second digit (tens)
                group_word = "%s %s" % (TENS[digit - 2], group_word)
            elif group_position == 0:   # hundreds
                # third digit (hundreds)
                group_word = "%s %s %s" % (DIGITS[digit - 1], GROUPS[0], group_word)
 
            digit_count += 1
 
        if group_word:
            # append the appropriate suffix (ex. thousand, million, etc.)
            # the first group doesn't get a suffix
            if group_count > 1:
                group_word = "%s %s," % (group_word, GROUPS[group_count - 1])
 
            # add this group word to the overall word
            word = "%s %s" % (group_word, word)
        
        # on to the next group
        group_count += 1
 
    return word.rstrip(', ')