def print_str(self, value, justify_right=True):
        """Print a 4 character long string of values to the display. Characters
        in the string should be any ASCII value 32 to 127 (printable ASCII).
        """
        # Calculcate starting position of digits based on justification.
        pos = (4-len(value)) if justify_right else 0
        # Go through each character and print it on the display.
        for i, ch in enumerate(value):
            self.set_digit(i+pos, ch)
Print a 4 character long string of values to the display. Characters
        in the string should be any ASCII value 32 to 127 (printable ASCII).