Beginner question

[eschneider92]

Why won't the 'goodbye' part of this code work right? it prints 'ok' no matter what is typed. Much thanks.

def thing():
print('go again?')
goagain=input()
if goagain=='y' or 'yes':
print('ok')
elif goagain!='y' or 'yes':
print('goodbye')
sys.exit()
thing()

 

[Chris Down]

Why won't the 'goodbye' part of this code work right? it prints 'ok' no
matter what is typed. Much thanks.

"if" statements do not fall through, because the first statement was matched,
no other ones in the same chain will be evaluted.

"elif" means "else if", where "else" means "if nothing previous matched".

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[Dave Angel]

Why won't the 'goodbye' part of this code work right? it prints 'ok' no matter what is typed. Much thanks.

def thing():
print('go again?')
goagain=input()
if goagain=='y' or 'yes':

This expression doesn't do what you think. The comparison binds more
tightly, so it first evaluates (goagain=="y"). The results of that are
either True or False. Then it or's that logical value with 'yes'. The
result is either True or it's 'yes'. 'yes' is considered truthy, so
the if will always succeed.

What you meant to use was:
if goagain == "y" or goagain == "yes":

Alternatively, you could use
if goagain in ("y", "yes"):

print('ok')
elif goagain!='y' or 'yes':

Same here.

 

[Chris Angelico]

Why won't the 'goodbye' part of this code work right? it prints 'ok' no matter what is typed. Much thanks.

def thing():
print('go again?')
goagain=input()
if goagain=='y' or 'yes':
print('ok')
elif goagain!='y' or 'yes':
print('goodbye')
sys.exit()
thing()

When you use 'or' in this way, it's not doing what you think it does.
It actually first computes

which is either True or False, and then evaluates the next part:

Try out those two in interactive Python and see what they do. You
probably want to compare in both halves of the test, or possibly use
the 'in' operator:

if goagain in ('y', 'yes'):

Also, you don't need an 'elif' there; just use a plain else. Repeating
the condition just introduces the chance of bugs - for instance, would
you notice the error in this?

if goagain=='y' or goagain=='yes':
print('ok')
elif goagain!='y' or goagain!='yes':
print('goodbye')

Using a plain 'else' guarantees that exactly one of the branches will
be executed, rather than having the possibility that neither will,
which isn't the intention here.

Hope that helps!

ChrisA

 

[eschneider92]

Thanks that helped a lot!