Sudokue checker

[freeze131]

I am trying to create a sudoku checker as my second project. Its not going well :D. So far i've got it to check for rows that are 9 digits in length - i'm not sure if that's correct..

puzzle = []

row = raw_input()
while row:
  puzzle.append(row)
  row = raw_input()

for row in xrange(9):
  seen = {}
  for column in xrange(9):
    digit = puzzle[column][row]
    if digit in seen:
      print 'duplicate', digit, 'in row', row
    else:
      seen[digit] = 1

I want it to check for columns and 3x3 grids effectively, someone PLEASE HELP