Python: Maximum recursion depth while getting the str of an object

Question

I'm making a program to get the amount of letters in a number:

def convert(number):
    lettercount = 0

    numstr = str(number)

    # One's places
    if len(numstr) is 1:
        if number == 1 or number == 2 or number == 6:
            lettercount += 3

        elif number == 4 or number == 5 or number == 9:
            lettercount += 4

        else:
            lettercount += 5

    # Ten's places
    elif len(numstr) is 2:
        if number == 10:
            lettercount += 3

        elif number == 11 or number == 12:
            lettercount += 6

        elif number == 15 or number == 16:
            lettercount += 7

        elif number == 13 or number == 14 or number == 19:
            lettercount += 8

        elif number == 17 or number == 18:
            lettercount += 9

        elif number == 20 or number == 30 or number == 40 or\
            number == 80 or number == 90:
            lettercount += 6

        else:
            lettercount += convert(int((numstr)[-1]))
            lettercount += convert(int(round(number, -1)))

    return lettercount

print "88 has %i letters in its name." % convert(88)
print "23 has %i letters in its name." % convert(23)
print "46 has %i letters in its name." % convert(46)

It works just fine and returns a correct response for the 88 and 23, but it gives a recursion depth error on 46. I'm confused; why does it happen on just 46?

---

Fixed code:

def convert(number):
    lettercount = 0

    numstr = str(number)

    # One's places
    if len(numstr) == 1:
        if number == 1 or number == 2 or number == 6:
            lettercount += 3

        elif number == 4 or number == 5 or number == 9:
            lettercount += 4

        else:
            lettercount += 5

    # Ten's places
    elif len(numstr) == 2:
        if number == 10:
            lettercount += 3

        elif number == 40 or number == 50:
            lettercount += 5

        elif number == 11 or number == 12 or number == 20 or number == 30 or\
            number == 80 or number == 90:
            lettercount += 6

        elif number == 15 or number == 16:
            lettercount += 7

        elif number == 13 or number == 14 or number == 19:
            lettercount += 8

        elif number == 17 or number == 18:
            lettercount += 9

        else:
            lettercount += convert(int((numstr)[-1]))
            lettercount += convert((int(numstr) // 10) * 10)

    return lettercount

print "88 has %i letters in its name." % convert(88)
print "23 has %i letters in its name." % convert(23)
print "46 has %i letters in its name." % convert(46)

Solution 1

Because when you do

convert(int(round(number, -1)))

you are calling convert(50). Since 50 isn't covered by your if statements, it gets to the else again, and calls convert(50) again, and so forth.

Solution 2

The problem here is that round(46, -1) will produce the value 50. When convert is called with the value 50 it will go to the exact same line

lettercount += convert(int(round(number, -1)))

The round(50, -1) call will produce 50 and at this point the convert function will execute infinitely