4x4 Membrane Keypad RPI

import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)

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

ROW = [5,6,13,19]
COL = [26,16,20,21]

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()