A sudoku solver

This method checks one of the non-whitespace characters in the input for validity. It returns 0 for ".", and the equivalent integer for digits. All other values fail.

# - - -   S u d o k u S o l v e r . _ _ r e a d C h a r   - - -

    def __readChar ( self, c ):
        """Translate one input character.

          [ c is a one-character string ->
              if c is "." ->
                return 0
              else if c is a digit in ["1", "9"] ->
                return int(c)
              else -> raise ValueError ]
        """
        if  c == ".":
            return 0
        elif "1" <= c <= "9":
            return int(c)
        else:
            raise ValueError, ( "Invalid sudoku character: '%s'" %
                                c )

There is no trace table for this method. Verification is by inspection.