I would like to check for valid input when the user inputs a choice between 'r', 'p', 's'
How would I go about doing this? And where in my code do I put it?

I've already tried adding this in the beginning and it didn't work:

```
possible_choices = ["r", "p", "s"]

def play():
    user = input(
        "What's your choice? 'r' for rock, 'p' for paper, 's' for scissors: "
    ).lower
    computer = random.choice(["r", "p", "s"])
    if user not in possible_choices:
        print('Uh oh! Please select a valid option', play())

```

entire code:

```
import random

game_loop = True

def play():
    user = input(
        "What's your choice? 'r' for rock, 'p' for paper, 's' for scissors: "
    ).lower
    computer = random.choice(["r", "p", "s"])

    if user == computer:
        return f"Tie! Computer chose {computer}"

    # r > s, s > p, p > r
    if win_or_lose(user, computer):
        return f"You won! Computer chose {computer}"
    return f"You lost! Computer chose {computer}"

def win_or_lose(player, opponent):

    if (
        (player == "r" and opponent == "s")
        or (player == "s" and opponent == "p")
        or (player == "p" and opponent == "r")
    ):
        return True

def play_again():
    p_a = input("Play again? (y/n): ")
    if p_a == "y":
        print(play())
    elif p_a == "n":
        quit()
    else:
        print("oof! Please choose correct option: (y/n)")
        play_again()

print(play())

while game_loop == True:
    play_again()

```