The challenge

Create a function that transforms any positive number to a string representing the number in words. The function should work for all numbers between 0 and 999999.

Examples:

number2words(0)  ==>  "zero"
number2words(1)  ==>  "one"
number2words(9)  ==>  "nine"
number2words(10)  ==>  "ten"
number2words(17)  ==>  "seventeen"
number2words(20)  ==>  "twenty"
number2words(21)  ==>  "twenty-one"
number2words(45)  ==>  "forty-five"
number2words(80)  ==>  "eighty"
number2words(99)  ==>  "ninety-nine"
number2words(100)  ==>  "one hundred"
number2words(301)  ==>  "three hundred one"
number2words(799)  ==>  "seven hundred ninety-nine"
number2words(800)  ==>  "eight hundred"
number2words(950)  ==>  "nine hundred fifty"
number2words(1000)  ==>  "one thousand"
number2words(1002)  ==>  "one thousand two"
number2words(3051)  ==>  "three thousand fifty-one"
number2words(7200)  ==>  "seven thousand two hundred"
number2words(7219)  ==>  "seven thousand two hundred nineteen"
number2words(8330)  ==>  "eight thousand three hundred thirty"
number2words(99999)  ==>  "ninety-nine thousand nine hundred ninety-nine"
number2words(888888)  ==>  "eight hundred eighty-eight thousand eight hundred eighty-eight"

The solution in Python code

Option 1:

words = "zero one two three four five six seven eight nine" + \
" ten eleven twelve thirteen fourteen fifteen sixteen seventeen eighteen nineteen twenty" + \
" thirty forty fifty sixty seventy eighty ninety"
words = words.split(" ")

def number2words(n):
    if n < 20:
        return words[n]
    elif n < 100:
        return words[18 + n // 10] + ('' if n % 10 == 0 else '-' + words[n % 10])
    elif n < 1000:
        return number2words(n // 100) + " hundred" + (' ' + number2words(n % 100) if n % 100 > 0 else '')
    elif n < 1000000:
        return number2words(n // 1000) + " thousand" + (' ' + number2words(n % 1000) if n % 1000 > 0 else '')

Option 2:

d={0:'zero',1:'one',2:'two',3:'three',4:'four',5:'five',
6:'six',7:'seven',8:'eight',9:'nine',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',
100:'hundred',1000:'thousand'}
def number2words(n):
    """ works for numbers between 0 and 999999 """
    if 0<=n<=20:return d[n]
    if 20<n<=99 and n%10:return d[10*(n//10)]+'-'+d[n%10]
    if 20<n<99:return d[10*(n//10)]
    if n<1000 and n%100==0:return d[n//100]+' '+d[100]
    if 100<n<=999:return d[n//100]+' '+d[100]+' '+number2words(n%100)
    if n%1000==0:return d[n//1000]+' '+d[1000]
    return number2words(n//1000)+' '+d[1000]+' '+number2words(n%1000)

Option 3:

d = {1: 'one', 2: 'two', 3: 'three', 4: 'four', 5: 'five',
     6: 'six', 7: 'seven', 8: 'eight', 9: 'nine', 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', 0:''}

def number2words(n):   
    s = (htu(n // 1000) + ' thousand ' if n // 1000 else '') + htu(n % 1000)
    return ' '.join(s.split()) if s else 'zero'
    
def htu(n):
    h, tu, u = n//100, n % 100, n % 10
    t = (d[tu] if tu in d else d[tu//10*10] + '-' + d[u]).strip('-')
    return d[h] + ' hundred ' + t if h else t

Test cases to validate our solution

from solution import number2words
import test

@test.describe("fixed tests")
def _():
    @test.it("One digit Tests")
    def _():
        test.assert_equals(number2words(0), "zero")
        test.assert_equals(number2words(1), "one")
        test.assert_equals(number2words(8), "eight")
        
    @test.it("Tens Tests")
    def _():
        test.assert_equals(number2words(10), "ten")
        test.assert_equals(number2words(19), "nineteen")
        test.assert_equals(number2words(20), "twenty")
        test.assert_equals(number2words(22), "twenty-two")
        test.assert_equals(number2words(54), "fifty-four")
        test.assert_equals(number2words(80), "eighty")
        test.assert_equals(number2words(98), "ninety-eight")
        
    @test.it("Hundreds Tests")
    def _():
        test.assert_equals(number2words(100),"one hundred")
        test.assert_equals(number2words(301),"three hundred one")
        test.assert_equals(number2words(793),"seven hundred ninety-three")
        test.assert_equals(number2words(800),"eight hundred")
        test.assert_equals(number2words(650),"six hundred fifty")
        
    @test.it("Thousands Tests")
    def _():
        test.assert_equals(number2words(1000), "one thousand")
        test.assert_equals(number2words(1003), "one thousand three")