Python - Error Checking

Question

Say I have a code snippet:

players_chosen_hit = int(input('Where do you want to try to hit the AI?: 1-9  '))

What if the user enters a letter? How can I handle it such that it will tell the user he screwed up and have him re-enter until he gets a number in?

How about this:

possibleusershipplaces = [1,2,3,4,5,6,7,8,9]

players_chosen_hit = int(input('Where do you want to try to hit the AI?: 1-9  '))

while players_chosen_hit not in possibleusershipplaces:
    players_chosen_hit = input('Please tell me where the hit is: 1-9  (or Ctrl/Command+C to quit) ')

players_chosen_hit = int(players_chosen_hit)

Answer 1

possibleusershipplaces = {1,2,3,4,5,6,7,8,9}
while True:
    try:
        players_chosen_hit = int(input('Where do you want to try to hit the AI?: 1-9  '))
        if players_chosen_hit in possibleusershipplaces:
            break
    except ValueError:
        print("Invalid entry")

To also handle quitting:

while True:
    try:
        players_chosen_hit = input('Please tell me where the hit is: 1-9  (q to quit) ')
        if players_chosen_hit == "q":
            print("Goodbye")
            break   
        players_chosen_hit = int( players_chosen_hit)
        if players_chosen_hit in possibleusershipplaces:
            break
    except ValueError:
        print("Invalid entry")

If you don't want a try/except and have only positive numbers you can use str.isdigit but the try/except is the idiomatic way:

possibleusershipplaces = {"1","2","3","4","5","6","7","8","9"}

for players_chosen_hit  in iter(lambda:input('Please tell me where the hit is: 1-9  (q to quit) '),"q"):
    if players_chosen_hit.isdigit() and players_chosen_hit in possibleusershipplaces:
        players_chosen_hit = int(players_chosen_hit)

iter takes a second argument sentinel which will break the loop if entered.

It might also be better to use a function and to return when we reach a condition:

def get_hit():
    while True:
        try:
            players_chosen_hit = input('Please tell me where the hit is: 1-9  (q to quit) ')
            if players_chosen_hit == "q":
                print("Goodbye")
                return   
            players_chosen_hit = int(players_chosen_hit)
            if players_chosen_hit in possibleusershipplaces:
                return players_chosen_hit
        except ValueError:
            print("Invalid entry")