While loop with "or"? Please help!

Bruno Desthuilliers bruno.desthuilliers at websiteburo.com
Thu Jan 25 13:31:25 CET 2007

wd.jonsson at gmail.com a écrit :
> Hmm, my while loop with "or" doesn't seem to work as I want it to...
> How do I tell the while loop to only accept "Y" or "y" or "N" or "n"
> input from the str(raw_input)?
> Thank's in advance!
> Snippet of code:
> import os
> def buildfinder():
>     os.system("CLS")
>     GameRoot = os.getenv("GAME_ROOT") + "\\"
>     print "Do you want to use " + GameRoot + " as your source
> directory?"
>     usr = str(raw_input('Y/N: '))
>     return usr
> #Runs the buildfinder function
> usrinp = buildfinder()
> def buildwhiler():
>     while usrinp != "y" or "Y" or "N" or "n": <<<<----PROBLEM

Yes, obviously.

The above reads as:
       while (usrinp != "y") or ("Y") or ("N") or ("n"):

What you want is something like:
      while usrinp != "y" and usrinp != "Y" \
            and usrinp != "n" and usrinp != "N" :

which is better written as
      while not (usrinp == "y" or usrinp == "Y" \
                 or usrinp == "n" or usrinp == "N") :

which can be simplified using str.lower:
      while not (usrinp.lower() == "y" or usrinp.lower() == "n"):

and simplified again thanks to Python 'in' operator:
     while not usrinp.lower() in "yn":

>         print "Enter Y or N!"
>         usr = str(raw_input('Y/N: '))
>     else:
>         code continues

While we're at it, there are other problems in your code. One of them is 
that buildwhiler() will go in an infinite loop if the usr first typed 
anything else than "y", "Y", "n" or "N". I let you try to find out why, 
and just give you two hints :
- globals are Bad(tm)
- don't mix application logic with user-interface logic (ie: a function 
should only deal with one or the other, not mix both).


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