John Machin wrote:
> On Aug 3, 8:55 am, Ian Clark <[EMAIL PROTECTED]> wrote:
>> Stef Mientki wrote:
>>> hello,
>>> I discovered that boolean evaluation in Python is done "fast"
>>> (as soon as the condition is ok, the rest of the expression is ignored).
>>> Is this standard behavior or is there a compiler switch to turn it on/off ?
>>> thanks,
>>> Stef Mientki
>> It's called short circuit evaluation and as far as I know it's standard
>> in most all languages. This only occurs if a conditional evaluates to
>> True and the only other operators that still need to be evaluated are
>> 'or's or the condition evaluates to False and all the other operators
>> are 'and's. The reason is those other operators will never change the
>> outcome: True or'd with any number of False's will still be True and
>> False and'ed to any number of Trues will still be False.
>>
>> My question would be why would you *not* want this?
>>
>>
>
> Why? Perhaps under some compound condition like this:
>
> (you_are_confused and/or
> function_returns_bool_but_has__side_effects())
>
Thanks guys,
Yes this is exactly what's going wrong ...
Sorry, my question missed the essential "NOT",
here is an example, that behaves different in Delphi,
(so I guess Delphi is not a real language ;-)
<Python>
def Some_Function (const):
print 'Ive been here', const
return True
A = True
if A and Some_Function (4 ):
print 'I knew it was True'
else:
print 'I''ll never print this'
</Python>
<Output>
Ive been here 4
I knew it was True
</Output
I was expected that the function would not be called,
because A is True.
And I agree with Laurent that it should be better to write a clean code,
so it doesn't matter whether you write in Python or in Delphi.
Gabriel: you pointed me to this page:
The exact behavior is defined in the Language Reference
<http://docs.python.org/ref/Booleans.html>
<quote>
or_test ::= and_test | or_test "or" and_test
</quote
Can you imagine, while I'm not a programmer, just a human,
that I don't understand one bit of this line.
So now I'm left with just one question:
for bitwise operations I should use &, |, ^
for boolean operations I should use and, or, xor
but after doing some test I find strange effects:
>>> A = 4
>>> B = 5
>>> A and B
5
>>> A & B
4
>>> A or B
4
>>> A | B
5
So if I use the bitwise operation on integers,
"and" changes into (bitwise) "or" and vise versa.
Is there some way to prevent / detect these kind of errors
( as I'm building a micro-controller simulator in Python,
I need both logical and bitwise operators very frequently).
cheers,
Stef Mientki
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