[Edu-sig] Interesting "gotcha"

[John Zelle]

Hi,

I agree that the behavior of booleans in Python can sometimes lead to subtle errors, but I think it's important to stress to students that writing things like:

if x>y == True:

is _really_ bad style. After pointing that out, I start taking points away for doing this. Code like this shows that the writer really does not understand Boolean types and their use. Expressions like this can cause subtle errors in just about any language, even those that are statically typed to prevent this sort of confusion. In Java, (x>y==true) is not a valid expression, so it won't compile. But consider this loop that seems to use done as a boolean flag::

boolean done = false;
while(done = false){
    // do some stuff here
}

The loop body will never execute. This sort of error in a strongly typed language is often very hard to spot, since we assume the compiler should catch it. If the condition is simply written as:

while(!done){

then you can't go wrong. 

My point is that while I agree this is a "gotcha" in Python, there are similar gotchas in just about all languages. The real culprit in the original example is poor coding style, not the language semantics per se.

--John

________________________________________
From: edu-sig-bounces+john.zelle=wartburg.edu at python.org [edu-sig-bounces+john.zelle=wartburg.edu at python.org] on behalf of Christian Mascher [christian.mascher at gmx.de]
Sent: Tuesday, March 29, 2011 10:12 AM
To: edu-sig at python.org
Subject: Re: [Edu-sig] Interesting "gotcha"

Hi,

I really like it that python allows me to write

if 10 < x <=  b:
     ...

impossible in many other languages.
But all things come at a cost:

>
>   if x > y == True:
>

I think this is a real gotcha, because it might get you, because you
_know too much_:
 >>> if 4:
        print True


True
 >>>
So all things different from 0, '', ... are considered True by python.
But.....

spoiler ahead ;-)
 >>> 4==True

False
 >>>

Cheers

Christian
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