Boolean Operator Confusion

Steven D'Aprano steve+comp.lang.python at pearwood.info
Fri Apr 24 17:14:52 CEST 2015


On Sat, 25 Apr 2015 12:50 am, subhabrata.banerji at gmail.com 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




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