how can i get the monster to show up

import random

CELLS = [(0, 0), (0, 1), (0, 2), (0, 3),
         (1, 0), (1, 1), (1, 2), (1, 3),
         (2, 0), (2, 1), (2, 2), (2, 3),
         (3, 0), (3, 1), (3, 2), (3, 3)]


def get_locations():
  player = random.choice(CELLS)
  monster = random.choice(CELLS)
  gate = random.choice(CELLS)

  if player == monster or gate == monster or player == gate:
    player, monster, gate = get_locations()

  return player, monster, gate

def get_m_locations(player,monster,gate):
  monster1 = random.choice(CELLS)

  if player == monster1 or gate == monster1 or monster == monster1:
     monster1 = get_m_locations(player,monster,gate)

  return monster1

def draw_map(pos,p_pos,m_pos,g_pos):
  print(" _ _ _ _")

  for num, item in enumerate(CELLS):
    if num not in [3,7,11,15]:
      if item == p_pos:
        print("|X",end="")
      elif item == m_pos:
        print("|M",end="")
      elif item == g_pos:
        print("|G",end="")
      else:
        if item in pos.values():
          for idx,item1 in pos.items():
            if item == item1:
              print("|{}".format(idx),end="")
              break
        else:
          print("|_",end="")

    else:
      if item == p_pos:
        print("|X|")
      elif item == m_pos:
        print("|M|",)
      elif item == g_pos:
        print("|G|",)        
      else:
        if item in pos.values():
          for idx,item1 in pos.items():
            if item == item1:
              print("|{}|".format(idx))
              break
        else:
          print("|_|")


def avail_moves(p_pos):
  x1,x2 = p_pos
  st = "Press "
  if not x1 == 0:
    st = st + "W "
  if not x1 == 3:
    st = st + "S "
  if not x2 == 0:
    st = st + "A "
  if not x2 == 3:
    st = st + "D "
  return st

def update_pos(p_pos,p_key):
  x1,x2 = p_pos
  if p_key == "W" and not x1 == 0:
    x1 -= 1
  elif p_key == "S" and not x1 == 3:
    x1 += 1
  elif p_key == "A" and not x2 == 0:
    x2 -= 1
  elif p_key == "D" and not x2 == 3:
    x2 += 1
  else:
    print("ERROR CHOICE : Enter again")
  return x1,x2

print("Welcome to the dungeon")
print("Get to the door to win while avoiding the monster")
print("The controls are W/S/A/D for UP/DOWN/LEFT/RIGHT")

p_pos,m_pos,g_pos = get_locations()

num = 0
pos = {num:p_pos}

while True:

  #check if game is over
  if p_pos == m_pos:
    print("Sorry.. You have been gobbled by the monster")
    break
  elif p_pos == g_pos:
    print("Yay.. You win.. You found the gate in {} steps".format(num))
    break
  else:
    draw_map(pos,p_pos,m_pos,g_pos)
    num += 1
    print("You are in cell {}".format(p_pos))
    print(avail_moves(p_pos))
    p_key = input("> ").upper()
    p_pos = update_pos(p_pos,p_key)
    pos.update({num:p_pos})