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Groups > comp.lang.python > #25373
| From | Terry Reedy <tjreedy@udel.edu> |
|---|---|
| Subject | Re: Implicit conversion to boolean in if and while statements |
| Date | 2012-07-15 16:28 -0400 |
| References | <mailman.2132.1342341291.4697.python-list@python.org> <5002a1f9$0$29995$c3e8da3$5496439d@news.astraweb.com> <5002F7AD.1000209@gmail.com> |
| Newsgroups | comp.lang.python |
| Message-ID | <mailman.2153.1342384095.4697.python-list@python.org> (permalink) |
On 7/15/2012 1:02 PM, Andrew Berg wrote:
> On 7/15/2012 5:56 AM, Steven D'Aprano wrote:
>> 3) Rather than distinguishing "true" from "false", a more useful
>> dichotomy is between "something" and "nothing". Python includes a number
>> of ways of spelling "nothing" of various types, such as:
>>
>> None, 0, 0.0, '', [], {}, set()
>>
>> and nearly everything else is "something".
> Okay, I see the value in this, but I don't understand why None has a
> truth value.
Because everything does (or should).
> I would expect None to mean "doesn't exist" or "unknown" or
> something like that - e.g., a value of 0 means 0 jelly beans in the jar
> and None means there isn't a jar.
>
> FWIW, I have, for a reason I forget, gotten into the habit of writing
> "if x is not None" when testing for None.
If x might possibly be any other false value (as is, I think, the usual
case), that is the right thing to do. Even if no other false value is
possible, I would still use 'is not None' just to be clear what the
false alternative is, and to guard against mistakes (like not knowing
that time values can be false) or code changes that add the possibility
of ohter false values.
> However, I have not been
> writing "if x is True: ..."/"elif x is False: ..."/"else: 'ruh-roh'"
> when testing for True (in cases where a value of True or False makes
> sense, but any other value would not). Should I?
If you only want to execute the if branch when x is literally the bool
object True and x could be some other non-bool true value such as 1 or
'a' or [1], etc, then you should.
If x is guaranteed to be True or False, which is the case you more or
less proposed, then you should not. For instance, "if isinstance(x, int)
is True:" or "if (n > 3) is True:" are redundant.
--
Terry Jan Reedy
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Implicit conversion to boolean in if and while statements Andrew Berg <bahamutzero8825@gmail.com> - 2012-07-15 03:34 -0500
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-15 10:56 +0000
Re: Implicit conversion to boolean in if and while statements Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-15 10:19 -0600
Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2012-07-15 09:50 -0700
Re: Implicit conversion to boolean in if and while statements Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-15 12:01 -0600
Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2012-07-15 11:56 -0700
Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2012-07-16 07:53 +1000
Re: Implicit conversion to boolean in if and while statements Ranting Rick <rantingrickjohnson@gmail.com> - 2012-07-15 18:21 -0700
Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2012-07-16 11:51 +1000
Re: Implicit conversion to boolean in if and while statements Ranting Rick <rantingrickjohnson@gmail.com> - 2012-07-15 19:31 -0700
Re: Implicit conversion to boolean in if and while statements Albert van der Horst <albert@spenarnc.xs4all.nl> - 2012-07-16 17:57 +0000
Re: Implicit conversion to boolean in if and while statements Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-07-17 00:44 -0400
Re: Implicit conversion to boolean in if and while statements Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-07-15 22:15 -0400
Re: Implicit conversion to boolean in if and while statements Ranting Rick <rantingrickjohnson@gmail.com> - 2012-07-15 19:58 -0700
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-16 04:03 +0000
Re: Implicit conversion to boolean in if and while statements Ranting Rick <rantingrickjohnson@gmail.com> - 2012-07-15 21:53 -0700
Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2012-07-16 15:57 +1000
Re: Implicit conversion to boolean in if and while statements alex23 <wuwei23@gmail.com> - 2012-07-16 00:21 -0700
Re: Implicit conversion to boolean in if and while statements Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-07-17 00:18 -0400
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-17 06:25 +0000
Re: Implicit conversion to boolean in if and while statements Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-07-17 05:38 -0400
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-16 02:58 +0000
Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2012-07-16 13:05 +1000
Re: Implicit conversion to boolean in if and while statements Ranting Rick <rantingrickjohnson@gmail.com> - 2012-07-15 20:21 -0700
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-16 04:20 +0000
Re: Implicit conversion to boolean in if and while statements Ranting Rick <rantingrickjohnson@gmail.com> - 2012-07-15 22:03 -0700
Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2012-07-16 16:00 +1000
Re: Implicit conversion to boolean in if and while statements alex23 <wuwei23@gmail.com> - 2012-07-16 00:27 -0700
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-16 07:52 +0000
Re: Implicit conversion to boolean in if and while statements Laszlo Nagy <gandalf@shopzeus.com> - 2012-07-16 23:26 +0200
Re: Implicit conversion to boolean in if and while statements Laszlo Nagy <gandalf@shopzeus.com> - 2012-07-16 23:17 +0200
Re: Implicit conversion to boolean in if and while statements Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-07-16 07:30 +0100
Re: Implicit conversion to boolean in if and while statements Albert van der Horst <albert@spenarnc.xs4all.nl> - 2012-07-16 18:03 +0000
Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2012-07-15 11:56 -0700
Re: Implicit conversion to boolean in if and while statements Serhiy Storchaka <storchaka@gmail.com> - 2012-07-16 16:01 +0300
Re: Implicit conversion to boolean in if and while statements rusi <rustompmody@gmail.com> - 2012-07-16 18:45 -0700
Re: Implicit conversion to boolean in if and while statements Rick Johnson <rantingrickjohnson@gmail.com> - 2012-07-15 09:50 -0700
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-16 02:13 +0000
Re: Implicit conversion to boolean in if and while statements Ranting Rick <rantingrickjohnson@gmail.com> - 2012-07-15 19:41 -0700
Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2012-07-16 12:58 +1000
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-16 08:05 +0000
Re: Implicit conversion to boolean in if and while statements Ethan Furman <ethan@stoneleaf.us> - 2012-07-16 11:13 -0700
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-17 06:14 +0000
Re: Implicit conversion to boolean in if and while statements Andrew Berg <bahamutzero8825@gmail.com> - 2012-07-15 12:02 -0500
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-16 02:38 +0000
Re: Implicit conversion to boolean in if and while statements Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2012-07-16 13:28 -0400
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-17 01:12 +0000
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-17 01:13 +0000
Re: Implicit conversion to boolean in if and while statements Andrew Berg <bahamutzero8825@gmail.com> - 2012-07-16 15:33 -0500
Re: Implicit conversion to boolean in if and while statements Ethan Furman <ethan@stoneleaf.us> - 2012-07-16 13:54 -0700
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-17 00:43 +0000
Re: Implicit conversion to boolean in if and while statements Andrew Berg <bahamutzero8825@gmail.com> - 2012-07-16 20:57 -0500
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-17 04:39 +0000
Re: Implicit conversion to boolean in if and while statements Andrew Berg <bahamutzero8825@gmail.com> - 2012-07-17 00:19 -0500
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-17 07:08 +0000
Re: Implicit conversion to boolean in if and while statements Andrew Berg <bahamutzero8825@gmail.com> - 2012-07-17 03:23 -0500
Re: Implicit conversion to boolean in if and while statements alex23 <wuwei23@gmail.com> - 2012-07-17 18:35 -0700
Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2012-07-17 18:32 +1000
Re: Implicit conversion to boolean in if and while statements Laszlo Nagy <gandalf@shopzeus.com> - 2012-07-17 14:43 +0200
Re: Implicit conversion to boolean in if and while statements Laszlo Nagy <gandalf@shopzeus.com> - 2012-07-17 14:59 +0200
Re: Implicit conversion to boolean in if and while statements Terry Reedy <tjreedy@udel.edu> - 2012-07-17 13:57 -0400
Re: Implicit conversion to boolean in if and while statements Ethan Furman <ethan@stoneleaf.us> - 2012-07-17 05:35 -0700
Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2012-07-17 12:13 +1000
Re: Implicit conversion to boolean in if and while statements Andrew Berg <bahamutzero8825@gmail.com> - 2012-07-15 12:16 -0500
Re: Implicit conversion to boolean in if and while statements Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-15 11:45 -0600
Re: Implicit conversion to boolean in if and while statements Terry Reedy <tjreedy@udel.edu> - 2012-07-15 16:09 -0400
Re: Implicit conversion to boolean in if and while statements Terry Reedy <tjreedy@udel.edu> - 2012-07-15 16:28 -0400
Re: Implicit conversion to boolean in if and while statements Andrew Berg <bahamutzero8825@gmail.com> - 2012-07-16 20:22 -0500
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-17 04:11 +0000
Re: Implicit conversion to boolean in if and while statements Ranting Rick <rantingrickjohnson@gmail.com> - 2012-07-16 21:18 -0700
Re: Implicit conversion to boolean in if and while statements Chris Angelico <rosuav@gmail.com> - 2012-07-17 14:24 +1000
Re: Implicit conversion to boolean in if and while statements Andrew Berg <bahamutzero8825@gmail.com> - 2012-07-16 23:44 -0500
Re: Implicit conversion to boolean in if and while statements Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-17 04:47 +0000
Re: Implicit conversion to boolean in if and while statements John Nagle <nagle@animats.com> - 2012-07-17 00:26 -0700
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