Coding python Keypad 4x4 di Raspberry Pi

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BOARD)
GPIO.setwarnings(False)

MATRIX = [ [1,2,3,'A'],
           [4,5,6,'B'],
           [7,8,9,'C'],
           ['*',0,'#','D'] ]

ROW = [7,11,13,15]
COL = [12,16,18,22]

for j in range(4):
    GPIO.setup(COL[j], GPIO.OUT)
    GPIO.output(COL[j], 1)

for i in range(4):
    GPIO.setup(ROW[i], GPIO.IN, pull_up_down = GPIO.PUD_UP)


try:
    while (True):
        for j in range(4):
            GPIO.output(COL[j],0)

            for i in range(4):
                    if GPIO.input(ROW[i]) == 0:
                        print MATRIX[i][j]
                        while GPIO.input(ROW[i]) == 0:
                                pass

            GPIO.output(COL[j],1)

           


except KeyboardInterrupt:
    GPIO.cleanup()