Simple Sudoku solver from an interview question

sudoku.py

#!/usr/bin/env python
class MyException(Exception):
    pass

import sys

def same_row(i,j):
    return (i/9 == j/9)
def same_col(i,j):
     return (i-j) % 9 == 0
def same_block(i,j):
     return (i/27 == j/27 and i%9/3 == j%9/3)

def r(a):
    i = a.find('0')
    if i == -1:
        #sys.exit(a)
        raise MyException(a)

    excluded_numbers = set()
    for j in range(81):
        if same_row(i,j) or same_col(i,j) or same_block(i,j):
            excluded_numbers.add(a[j])

    for m in '123456789':
        if m not in excluded_numbers:
            r(a[:i]+m+a[i+1:])


def display(res):
    rowcount = 1
    colcount = 1

    print "-----------------------"
    for n in res:
        print n,
        if colcount % 3 == 0:
            print "|",
        if colcount % 9 == 0:
            print ""
            if rowcount % 3 == 0:
                print "-----------------------"
            rowcount += 1
        colcount += 1

def main():
    if len(sys.argv) == 2:
        filename = sys.argv[1]
        f = open(filename)
        sudoku = "".join([line.strip("\n").strip() for line in f.readlines()])
        f.close()
    if len(sudoku) != 81:
        sys.exit("File incorrectly formatted - must contain 81 digits")

    print "Input\n"
    display(sudoku)
    print "\n"
    try:
        r(sudoku)
    except MyException as e:
        print "Answer\n"
        display(str(e))
    else:
        print 'Usage: python sudoku.py puzzle'
        print 'where puzzle is an 81 char string representing the puzzle read left-to-right, top-to-bottom, and 0 is a blank'

if __name__ == '__main__':
    status = main()
    sys.exit(status)