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Re: Implicit conversion to boolean in if and while statements

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Newsgroups comp.lang.python
Date Sun, 15 Jul 2012 09:50:05 -0700 (PDT)
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Subject Re: Implicit conversion to boolean in if and while statements
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Xref csiph.com comp.lang.python:25358

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On Sunday, July 15, 2012 11:19:16 AM UTC-5, Ian wrote:
> On Sun, Jul 15, 2012 at 4:56 AM, Steven D'Aprano
> <steve+comp.lang.python@pearwood.info> wrote:
> > (For the record, I can only think of one trap for the unwary: time
> > objects are false at *exactly* midnight.)
> 
> Ugh, that's irritating.  I can't think of any scenario where I would
> ever want the semantics "if timeval (is not midnight):".  This
> reinforces the point that if you only want to test whether you have
> None, you should use "is not None" rather than relying on __bool__.

I think this issue is not so much a "bool test" vs "type test", but more an ambiguous syntax issue. Consider this:

## EXAMPLE A ##
py> if money:
...     do_something()

The syntax "if money" implies we are testing/measuring some attribute of "money", but what exactly about money are we testing/measuring? The problem lies in the syntax. To understand this syntax, we must first interpret what *IF* means, and we should *NEVER* need to interpret such a well defined word as *IF*! This syntax is far too opaque. Consider the alternative:

## EXAMPLE B ##
py> if bool(money):
...     do_something()

Now we have a hint. Even if we don't understand the inner workings of the "bool" function, we *do* understand that the statement "bool(money)" *must* return True or the block *will not* execute. 

We must NEVER present "if" in such confusing manner as ExampleA. I believe Guido made a grave mistake allowing this syntax to flourish. His intentions where noble, to save people a few keystrokes, but all he accomplished was to pave a road directly into hell. 

 "Explict is better than Implict"

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Thread

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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