Rajarshi wrote: > This is a slightly naive question, but I know that 0 can be used to > represent False. So
>>>> 0 == False > True
> But, I know I can use [] to represent False as in
>>>> if not []: print 'empty' > ... > empty
> But then doing the following gives a surprising (to me!) result
>>>> [] == False > False
> Could anybody point out why this is the case?
"if foo:" does not check if "foo == True" or "foo == False" but rather "bool(foo)". For empty lists, strings, tuples, dicts and some other things, "bool(foo) == False", while for lists, etc., with at least one element, "bool(foo) == True".
-- Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco
Writing, "if x" is short for writing "if bool(x)". Evaluating bool(x) checks for a x.__nonzero__() and if that method isn't defined, it checks for x.__len__() to see if x is a non-empty container.
In your case, writing "if []" translates to "if len([]) != 0", which evaluates to False.
True and False are of type bool which is a subclass of int. So, False really is equal to zero and True really is equal to one.
In contrast, the empty list is not of type int. So [] != False eventhough bool([]) == False.
> This is a slightly naive question, but I know that 0 can be used to > represent False. So
> >>> 0 == False
> True
> But, I know I can use [] to represent False as in
> >>> if not []: print 'empty'
> ... > empty
> But then doing the following gives a surprising (to me!) result
> >>> [] == False
> False
> Could anybody point out why this is the case?
> Thanks, > Rajarshi
This has *got* to rank up there among the VFAQ's of them all, along with the mysterious shared default empty list argument. I think this particular question has been asked in one form or another at least twice a week for the past month!
-- Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/ San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis Woman was God's _second_ mistake. -- Friedrich Nietzsche
Rajarshi <rajarshi.g...@gmail.com> wrote: >This is a slightly naive question, but I know that 0 can be used to >represent False. So
>>>> 0 == False >True
>But, I know I can use [] to represent False as in
>>>> if not []: print 'empty' >... >empty
>But then doing the following gives a surprising (to me!) result
>>>> [] == False >False
>Could anybody point out why this is the case?
False is just a constant. 0, (), '', [], and False are all constants that happen to evaluate to a false value in a Boolean context, but they are not all the same.
As a general rule, I've found code like "if x == False" to be a bad idea in ANY language. -- Tim Roberts, t...@probo.com Providenza & Boekelheide, Inc.
In article <1179983482.452458.188...@q66g2000hsg.googlegroups.com>, Paul McGuire <pt...@austin.rr.com> wrote:
> This has *got* to rank up there among the VFAQ's of them all, along > with the mysterious shared default empty list argument. I think this > particular question has been asked in one form or another at least > twice a week for the past month!
Anyone who finds this surprising, might enjoy reading this article from the time several years ago when the feature was being considered. When you have some time - it's long, but interesting. The present confusion is more directly addressed towards the end. Yes, it's the Laura Creighton article again:
Donn Cave wrote: > Anyone who finds this surprising, might enjoy reading this > article from the time several years ago when the feature > was being considered. When you have some time - it's long, > but interesting. The present confusion is more directly > addressed towards the end. Yes, it's the Laura Creighton > article again:
If so, be sure to click "More options," then "View thread," and then read the responses. There were many reasonable objections to her points.
-- Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/ San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis Human salvation lies in the hands of the creatively maladjusted. -- Dr. Martin Luther King, Jr.
On May 24, 1:59 am, Tim Roberts <t...@probo.com> wrote: ...
> False is just a constant. 0, (), '', [], and False are all constants that > happen to evaluate to a false value in a Boolean context, but they are not > all the same.
> As a general rule, I've found code like "if x == False" to be a bad idea in > ANY language.
I have a job as a C++ programmer, and they make us write it like that, apparently because the ! operator is hard to see. But "if (x == TRUE)" is discouraged.
Dan Bishop wrote: > On May 24, 1:59 am, Tim Roberts <t...@probo.com> wrote: > ... >> False is just a constant. 0, (), '', [], and False are all constants that >> happen to evaluate to a false value in a Boolean context, but they are not >> all the same.
>> As a general rule, I've found code like "if x == False" to be a bad idea in >> ANY language.
> I have a job as a C++ programmer, and they make us write it like that, > apparently because the ! operator is hard to see. But "if (x == > TRUE)" is discouraged.
Find a new employer. I'm not joking.
regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden ------------------ Asciimercial --------------------- Get on the web: Blog, lens and tag your way to fame!! holdenweb.blogspot.com squidoo.com/pythonology tagged items: del.icio.us/steve.holden/python All these services currently offer free registration! -------------- Thank You for Reading ----------------
>> I have a job as a C++ programmer, and they make us write it like that, >> apparently because the ! operator is hard to see. But "if (x == >> TRUE)" is discouraged.
> Find a new employer. I'm not joking.
Really. He's not. That's a perfect example of a style guideline that not only wastes energy, misses the point, but is totally wrong.
-- Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/ San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis There is another world, which is not of men. -- Li Bai
In article <WPqdnYlb6LfHcMjbnZ2dnUVZ_urin...@speakeasy.net>, Erik Max Francis <m...@alcyone.com> wrote:
> Donn Cave wrote:
> > Anyone who finds this surprising, might enjoy reading this > > article from the time several years ago when the feature > > was being considered. When you have some time - it's long, > > but interesting. The present confusion is more directly > > addressed towards the end. Yes, it's the Laura Creighton > > article again:
> If so, be sure to click "More options," then "View thread," and then > read the responses. There were many reasonable objections to her points.
Not that it is of no historical interest to review all these reasonable arguments, but allow me to restore the context quote from my follow-up:
In article <1179983482.452458.188...@q66g2000hsg.googlegroups.com>, Paul McGuire <pt...@austin.rr.com> wrote:
> This has *got* to rank up there among the VFAQ's of them all, along > with the mysterious shared default empty list argument. I think this > particular question has been asked in one form or another at least > twice a week for the past month!
Donn Cave wrote: > Not that it is of no historical interest to review all these > reasonable arguments, but allow me to restore the context quote > from my follow-up:
If the counterpoints are of no historical interest, then the original point must be of no historical interest either, since it was not widely granted as true.
-- Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/ San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis Fear is an emotion indispensible for survival. -- Hannah Arendt
In article <cd2dnRy_aJMF78HbnZ2dnUVZ_t7in...@speakeasy.net>, Erik Max Francis <m...@alcyone.com> wrote:
> Donn Cave wrote:
> > Not that it is of no historical interest to review all these > > reasonable arguments, but allow me to restore the context quote > > from my follow-up:
> If the counterpoints are of no historical interest, then the original > point must be of no historical interest either, since it was not widely > granted as true.
"Not that it is of no historical interest" may have been too hard to follow, my apologies. I should have said "It may be of historical interest ...". After that, you lost me, but I guess I'm not going to worry about it.
On Tue, 29 May 2007 11:36:07 -0700, Erik Max Francis wrote: > Donn Cave wrote:
>> Not that it is of no historical interest to review all these >> reasonable arguments, but allow me to restore the context quote >> from my follow-up:
> If the counterpoints are of no historical interest, then the original > point must be of no historical interest either, since it was not widely > granted as true.
I hope you don't get just as confused by expressions like:
if not x != 5
*wink*
In English, a double negative is usually a positive. So "Not that it is of no historical interest" means "It is of historical interest".
I wonder, if somebody with more time on their hands than me were to go through the threads on comp.lang.python before and after the introduction of bools, could we determine whether there were more problems caused by the introduction of True and False than by the lack of them? Although I like using bools, in my heart of hearts I suspect that Laura was right.
Donn Cave wrote: > "Not that it is of no historical interest" may have been too > hard to follow, my apologies.
Yeah, my reading comprehension wasn't up to snuff that night.
-- Erik Max Francis && m...@alcyone.com && http://www.alcyone.com/max/ San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis A man that studieth revenge keeps his own wounds green. -- Francis Bacon
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