How does Python interpret this statement as True?

Question

The following is a part of code RockPaperScissors, I can't figure out how Python, interpret

if answer in ("y", "yes", "Y", "Yes", "Of course"):
    return answer

since this return as a True.

and this other part of code below returns an Integer

player = int(player)
if player in (1, 2, 3):
    return player

How is it possible that it returns True for one part and an Integer in otherpart since it is the same structure?

based on the code below:

def start():
    print "Let's play a game of Rock, Paper, Scissors"
    while game():
        pass
    scores()

def game():
    player = move()
    computer = random.randint(1, 3)
    result(player, computer)
    return play_again()

def move():
    while True:
        print
        player = raw_input("Rock = 1\nPaper = 2\nScissors = 3\nMake a move : ")
        try:
            player = int(player)
            if player in (1, 2, 3):
                return player
        except ValueError:
            print "Oops ! I didn't understand that, Please enter 1, 2 or 3."

def result(player, computer):
    print "1..."
    time.sleep(1)
    print "2..."
    time.sleep(1)
    print "3 !"
    time.sleep(0.5)
    print "Computer threw {0}!".format(names[computer])
    global player_score, computer_score
    if player == computer:
        print "Tie game"
    else:
        if rules[player] == computer:
            print "victory getting closer !"
            player_score += 1

def play_again():
    answer = raw_input("Would you like to play again ?")
    if answer in ("y", "yes", "Y", "Yes", "Of course"):
        return True
    else:
        print "ok that's enough for today !"

def scores():
    global player_score, computer_score
    print "HIGH SCORES"
    print "Player : " + player_score
    print "Computer :" + computer_score

Why is that puzzling? The `answer in (...)` test is only true if the value in `answer` is also in that tuple. — Martijn Pieters, Jul 09 '15 at 15:31
Neither code returns `True`. It returns the value of `answer` in one case, the value of `player` in the other case. `play_again()` returns `None` if the `if` statement was never true; a non-empty string is considered true, and `None` is considered false. So `play_again()` either returns one of those strings (`'y'`, `'Y'`, etc.) or it returns `None`, and those two objects have different [truth values](https://docs.python.org/2/library/stdtypes.html#truth-value-testing). — Martijn Pieters, Jul 09 '15 at 15:55

score 4 · Answer 1 · answered Jul 09 '15 at 15:32

4

If answer is an element of ("y", "yes", "Y", "Yes", "Of course"), then that evaluates to True.

answered Jul 09 '15 at 15:32

cdonts

9,304
4
46
72

Martijn Pieters · Accepted Answer · 2015-07-09T16:29:15.453

It appears you think that return answer returns True. It does not.

It returns the value of answer, which will be a non-empty string equal to one of the strings in the tuple. So the function play_again() returns 'Y' if you typed that in, because 'Y' is in the tuple.

When answer has a value that is not in the list, the if suite is not executed, so the else: branch prints a message and the function ends without a return statement. Python then returns None instead.

So play_again() either returns a non-empty string, one that is a member of the tuple ("y", "yes", "Y", "Yes", "Of course"), or it returns None. game() returns that value unchanged, back to the while loop in start():

while game():
    pass

The while statement tests the truth value of the return value. A non-empty string is considered true (so the loop continues), and None is considered false (the loop ends).

score 1 · Answer 3 · answered Jul 09 '15 at 15:34

Based on user input (which could be yes or no) assigned to variable answer Python checks to see this variable appears in ("y", "yes", "Y", "Yes", "Of course").

It's a little syntactic sugar.

You could also do:

for x in ("y", "yes", "Y", "Yes", "Of course"):
    if answer == x:
        return answer

More info on the python operands (e.g. is, is not and not in) in the docs.

score 0 · Answer 4 · answered Jul 09 '15 at 15:36

0

answer in ("y", "yes", "Y", "Yes", "Of course") evaluates to True if answer is equal to any of ("y", "yes", "Y", "Yes", "Of course")'s elements.

answered Jul 09 '15 at 15:36

raymelfrancisco

828
2
11
21

How does Python interpret this statement as True?

4 Answers4