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: [email protected]
[[email protected]] on behalf of Christian
Mascher [[email protected]]
Sent: Tuesday, March 29, 2011 10:12 AM
To: [email protected]
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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