"y" or "yes" -- expectations not met

In the following code, answer() works as expected and returns true if input is "y" and false when not, but in answer2(), it always returns true.

Can someone explain why this is the case?

def answer():
    answer = raw_input()
    if answer == "y":
        return True
    else:
        return False


def answer2():
    answer = raw_input()
    if answer == "y" or "yes":  # <- notice the extra: or "yes"
        return True
    else:
        return False


if answer() == True:
    print "true"
else:
    开发者_运维问答print "false"


if answer2() == True:
    print "true"
else:
    print "false"

The expression, "y" or "yes" will evaluate to "y". What you want is:

if answer in ('y', 'yes'):
    return True

I'm not a Python expert, but I suspect in answer2 it should be:

if answer == "y" or answer == "yes":  # <- notice the extra: or "yes"

In other words, I suspect it's currently parsing it as:

if (answer == "y") or ("yes")

and just converting "yes" into True, effectively... which is why it's always returning True.

You want

if answer == "y" or answer == "yes":

you have to test it that way.

Or better still, have a list of affirmative responses and test inclusion;

affirmatives = ["yes","y","ja","oui"]
if answer in affirmatives:

It's all to do with boolean (True/False) operators. Check out the following:

>>> "yes" == "yes" or "fnord"
True
>>> "no" == "yes" or "fnord"
'fnord'

If the first part is True, you get True - if the first part is False, you get the right-hand side of the 'or'. Now, by having:

 answer == "y" or answer == "yes"

you can see how if the first == returns False it returns the second == test...

you need to say:

if answer == "y" or answer == "yes"

how you did it evaluates as:

if (answer == "y") or "yes"

which will always be true as "yes" is a non-empty string.

if answer == "y" or "yes":

Note that this is different that:

if answer == "y" or answer == "yes":

Because "yes" is a non-zero value, the first one will always return True.

Here's the issue:

answer == 'y' or 'yes' 

is always going to return True or 'yes' (effectively True since it is not 0 or None),because it's being evaluated as:

if (answer == 'y') or 'yes'

so you are always going to trigger the 'if' block and return True.