How to restrict the choice of a user in python?

Question

So I have this program where I need the user to choose a length for a key in python but I want the choice of the user to be restricted with the provided lengths only and I don't quite know how to do that here is the code

available_len = [5 , 10 , 14 , 12 , 15, 16 , 18 , 20 , 21]
length = int(input("[+]:Select the length you want for your key: "))

I want to make so that if the user inputs a length not available in the list to print "Please choose a valid length" and take him back to input the length

I have tried:

 while True:
        length = int(input("[+]:Select the length you want for your key: "))
        if 5 <= length <= 21:
            break

Answer 1

You can't control what the user inputs. You can control whether or not you ask for a different input.

Model this with an infinite loop and an explicit break statement.

available_len = [5 , 10 , 14 , 12 , 15, 16 , 18 , 20 , 21]
while True:
    length = int(input("[+]:Select the length you want for your key: "))
    if length in availble_len:
        break

While you can shorten this using an assignment expression, I find this approach to be clearer.

available_len = [5 , 10 , 14 , 12 , 15, 16 , 18 , 20 , 21]
while (length := int(input("[+]:Select the length you want for your key: "))) not in available_len:
    pass

You could replace pass with a print statement explaining why the previous choice was invalid.