为什么使用！=运算符会在我的python程序中给我一个简单的错误？

Question

Why using the != operator produces an error in my python program

def TakeTurn():

    time.sleep(1.5)
    turning = input("You have reached a junction.\nDo you want to turn left (L), turn right (R) or go straight ahead(S)?")
    if turning != "L" :
        print ("Sorry, I didn't understand that")
    elif turning != "R" :
        print ("Sorry, I didn't understand that")
    elif turning != "S" :
        print ("Sorry, I didn't understand that")
    else:
        choice = randint (1,10)

    print (choice)

If I enter the letter L or R or S it prints Sorry, I didn't understand that

Thank you in advance for the help !!!

Answer 1

Your logic is faulty, one of these ifs will always match, so it will always print I didn't understand.

If the letter is not 'L' the first if will trigger, if it is the second will trigger.

What you want is:

if turning != 'L' and turning != 'R' and turning != 'S':
    print "Sorry, I didn't understand that"

or better yet:

if turning in ['L', 'R', 'S']:
    # do stuff for turning
else:
    print "Sorry, I didn't understand that"

Answer 2

There is a flaw in your logic. If you enter the letter "L", the first if condition will evaluate to False, which is good, but then the next if condition will evaluate to True, which is certainly not what you want! If you think about it more carefully, any character you enter will always meet one of these conditions.

What you really want is the following:

if turning not in ["L", "R", "S"]:
    print ("Sorry, I didn't understand that")
else:
    choice = randint (1,10)
    print(choice)