Please help me understand why my tuple changes in the Python Basics Dungeon Game?

#Create a game with a 2-dimensional map. Place the player, a door, and a monster into random spots in your map. Let the player move around in the map and, after each move, tell them if they've found the door or the monster. If they find either the game is over. The door is the win condition, the monster is the lose condition.

import random

def show_help():
    print("Welcome to the Dungeon. Try to reach the door w/o running into the monster.")
    print("Enter 'UP', 'DOWN', 'LEFT', or 'RIGHT' to move around the map. \nEnter 'HELP' for this info")

def direction(inp, li):
    if inp == 'HELP':
        show_help()
    elif inp == 'UP':
        li[1] = li[1]-1
    elif inp == 'RIGHT':
        li[0] = li[0]+1
    elif inp == 'DOWN':
        li[1] = li[1]+1
    elif inp == 'LEFT':
        li[0] = li[0]-1
    else:
        print('NOT VALID')
    return li

def wall(li):
    if li[1] < 1:
        li[1] = li[1]+1
    elif li[0] > 6:
        li[0] = li[0]-1
    elif li[1] > 6:
        li[0] = li[0]-1
    elif li[0] < 1:
        li[1] = li[1]+1


columns = list(range(1,7))
rows = list(range(1,7))

gridlist = []
for column in columns:
    for row in rows:
        gridlist.append([column, row])
grid = tuple(gridlist)
print("gridin: {}".format(grid))

player = random.choice(gridlist)
gridlist.remove(player)  

door = random.choice(gridlist)
gridlist.remove(door)

monster = random.choice(gridlist)

show_help()

while player != door:
    if player == door:
        print("Congrats! You made it out alive!")
        break
    elif player == monster:
        print("You just got eaten!")
        break
    elif not player in grid:
        print("There's a wall here. Can't go this direction further")
        #player = wall(player)
        break
    else:
        print("playerin: {}".format(player))
        way = input("> ")
        player = direction(way, player)
        print("playerafter: {}".format(player))
        print("grid: {}".format(grid))
        print("gridlist: {}".format(gridlist))
        continue