I can't correctly track KEYDOWN Events in PyGame

I can't figure out how to correctly track key-down events in PyGame. Whenever I try to increase the player's coordinates plx or ply, it doesn't work and it prints the same over and over!

import pygame, sys
from pygame.locals import *

global plx
global ply

plx = 0
ply = 0

DISPLAYSURF = pygame.display.set_mode((1, 1))
pygame.display.set_caption('Text Only Jam')

while True:
    for event in pygame.event.get():
        if event.type == pygame.KEYDOWN:
            if pygame.K_LEFT:
                plx -= 1
                print(plx)
            if pygame.K_RIGHT:
                plx += 1
                print(plx)

        if event.type == QUIT:
            pygame.quit()
            sys.exit()

    pygame.display.update()

And, here is the output:

pygame 1.9.6
Hello from the pygame community. https://www.pygame.org/contribute.html
-1
0
-1
0
-1
0

I have also tried setting the variables in other ways, and I still can't get it to work. I have used some basic match like this before, so I don't know what to do. Any help is appreciated!