Boolean Operator Confusion

[subhabrata.banerji at gmail.com]

On Friday, April 24, 2015 at 8:45:04 PM UTC+5:30, Steven D'Aprano wrote:
> On Sat, 25 Apr 2015 12:50 am,wrote:
> 
> > Dear Group,
> > 
> > I am trying to understand the use of Boolean operator in Python. I am
> > trying to write small piece of script, as follows,
> > 
> > def input_test():
> >     str1=raw_input("PRINT QUERY:")
> >     if "AND" or "OR" or "NOT" in str1:
> >         print "It is a Boolean Query"
> >     elif "AND" or "OR" or "NOT" not in str1:
> >         print "It is not a Boolean Query"
> >     else:
> >         print "None"
> 
> First problem: why do you sometimes return "None"? You have two possible
> answers: either something is a boolean query, or it is not. There is no
> third choice. ("It's a boolean query, but only on Wednesdays.")
> 
> So your code you have if...else and no "elif" needed.
> 
>     if <test for boolean query>:
>         print "It is a Boolean Query"
>     else:
>         print "It is not a Boolean Query"
> 
> 
> Now let us look at your test for a boolean query:
> 
>     "AND" or "OR" or "NOT" in str1
> 
> 
> How does a human reader understand that as English?
> 
>     if "AND" is in the string, or "OR" is in the string, or "NOT" 
>     is in the string, then it is a boolean query.
> 
> But sadly, that is not how Python sees it. Python sees it as:
> 
>     if "AND" is a true value, or if "OR" is a true value, 
>     or if "NOT" is in the string, then it is a boolean query.
> 
> "AND" is always a true value. All strings except for the empty string "" are
> true values, so expressions like:
> 
>     if X or Y or Z in some_string
> 
> will always be true, if X or Y are true values.
> 
> You need to re-write your test to be one of these:
> 
>     # version 1
>     if "AND" in str1 or "OR" in str1 or "NOT" in str1:
>         print "It is a Boolean Query"
>     else:
>         print "It is not a Boolean Query"
> 
> 
>     # version 2
>     if any(word in str1 for word in ("AND", "OR", "NOT")):
>         print "It is a Boolean Query"
>     else:
>         print "It is not a Boolean Query"
> 
> 
> 
> 
> -- 
> Steven

Thanks Steven for taking so much of your valuable time to make me understand and nice guidance. Ian your suggestion worked well. Thank you. Regards, Subhabrata Banerjee.