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Groups > comp.lang.python > #65015

Re: 1 > 0 == True -> False

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Roy Smith writes:

> In article <3dcdc95d-5e30-46d3-b558-afedf9723c7c@googlegroups.com>,
>  Thibault Langlois wrote:
> 
> > You are right. I should have given some context.  I am looking at
> > this from the perspective of the teacher that has to explain
> > idiosyncrasies of the language to inexperienced students.
> >
> > There are two aspects in this example. 

> > 1. the equivalence of True/False with integers 1/0 which have pro
> > and cons.
> > 2. the chaining rules of operators. I agree that it may make sense
> > in some cases like x > y > z but when operators are mixed it leads
> > to counter intuitive cases as the one I pointed out.
> > 
> > The recommendations to student are 1) do not assume True == 1 and
> > do not use operator chaining.
> 
> Better than that, do what I do.
> 
> 1) Assume that you don't have the full operator precedence table
> memorized and just parenthesize everything.
> 
> 2) In cases where the expression is so simple, you couldn't possibly
> be wrong, see rule #1.

There's nothing to parenthesize in x <= y < z = w, unless you mean
something rather weird that is not equivalent anyway (and may not be
compatible with avoiding assumptions like True == 1).

There's not much to remember: the comparison operators are a
semantically coherent class (as far as I can see) and have the same
precedence level somewhere between proper operators (let's call them
that) and and and or and if else[1]. Ok, I'm not quite sure how the
two branches of a conditional expression combine. Rather than find
out, use parentheses, yes:

   (x + 1) if c else x ==  x + (1 if c else 0) == x + bool(c)

I agree about using parentheses when in doubt, but there is some room
here for more education and less doubt. Python gets these right.

[1] Sorry...

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Thread

1 > 0 == True -> False Thibault Langlois <thibault.langlois@gmail.com> - 2014-01-30 03:36 -0800
  Re: 1 > 0 == True -> False Thomas Mlynarczyk <thomas@mlynarczyk-webdesign.de> - 2014-01-30 12:44 +0100
  Re: 1 > 0 == True -> False Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-01-30 13:46 +0200
    Re: 1 > 0 == True -> False Peter Otten <__peter__@web.de> - 2014-01-30 13:04 +0100
      Re: 1 > 0 == True -> False Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-01-30 14:08 +0200
  Re:1 > 0 == True -> False Dave Angel <davea@davea.name> - 2014-01-30 07:49 -0500
    Re: 1 > 0 == True -> False Thibault Langlois <thibault.langlois@gmail.com> - 2014-01-30 05:40 -0800
      Re: 1 > 0 == True -> False Chris Angelico <rosuav@gmail.com> - 2014-01-31 00:55 +1100
      Re: 1 > 0 == True -> False Roy Smith <roy@panix.com> - 2014-01-30 09:08 -0500
        Re: 1 > 0 == True -> False Chris Angelico <rosuav@gmail.com> - 2014-01-31 01:18 +1100
          Re: 1 > 0 == True -> False Roy Smith <roy@panix.com> - 2014-01-30 09:49 -0500
            Re: 1 > 0 == True -> False Chris Angelico <rosuav@gmail.com> - 2014-01-31 02:02 +1100
        Re: 1 > 0 == True -> False Devin Jeanpierre <jeanpierreda@gmail.com> - 2014-01-30 06:41 -0800
        Re: 1 > 0 == True -> False Thibault Langlois <thibault.langlois@gmail.com> - 2014-01-30 06:46 -0800
          Re: 1 > 0 == True -> False Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-01-30 17:42 +0000
        Re: 1 > 0 == True -> False Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-01-30 16:56 +0200
          Re: 1 > 0 == True -> False Roy Smith <roy@panix.com> - 2014-01-30 10:46 -0800
            Re: 1 > 0 == True -> False Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2014-01-30 22:14 +0200
              Re: 1 > 0 == True -> False Chris Angelico <rosuav@gmail.com> - 2014-01-31 07:25 +1100
        Re: 1 > 0 == True -> False Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-01-30 15:09 +0000
          Re: 1 > 0 == True -> False Rustom Mody <rustompmody@gmail.com> - 2014-01-30 07:34 -0800
          Re: 1 > 0 == True -> False Roy Smith <roy@panix.com> - 2014-01-30 10:53 -0800
            Re: 1 > 0 == True -> False Rustom Mody <rustompmody@gmail.com> - 2014-01-30 19:33 -0800
          Re: 1 > 0 == True -> False Roy Smith <roy@panix.com> - 2014-01-30 10:56 -0800
            Re: 1 > 0 == True -> False Chris Angelico <rosuav@gmail.com> - 2014-01-31 06:03 +1100
              Re: 1 > 0 == True -> False Roy Smith <roy@panix.com> - 2014-01-30 14:09 -0800
                Re: 1 > 0 == True -> False Chris Angelico <rosuav@gmail.com> - 2014-01-31 09:29 +1100
            Re: 1 > 0 == True -> False Ethan Furman <ethan@stoneleaf.us> - 2014-01-30 11:22 -0800
            Re: 1 > 0 == True -> False Chris Angelico <rosuav@gmail.com> - 2014-01-31 06:48 +1100
    Re: 1 > 0 == True -> False Rotwang <sg552@hotmail.co.uk> - 2014-01-30 19:25 +0000
      Re: 1 > 0 == True -> False Dave Angel <davea@davea.name> - 2014-01-30 15:08 -0500
      Re: 1 > 0 == True -> False Chris Angelico <rosuav@gmail.com> - 2014-01-31 07:15 +1100
      Re: 1 > 0 == True -> False Ian Kelly <ian.g.kelly@gmail.com> - 2014-01-30 13:28 -0700
      Re: 1 > 0 == True -> False Chris Angelico <rosuav@gmail.com> - 2014-01-31 07:38 +1100
      Re: 1 > 0 == True -> False Ian Kelly <ian.g.kelly@gmail.com> - 2014-01-30 14:17 -0700
      Re: 1 > 0 == True -> False Chris Angelico <rosuav@gmail.com> - 2014-01-31 08:31 +1100
      Re: 1 > 0 == True -> False Joshua Landau <joshua@landau.ws> - 2014-01-30 23:36 +0000
        Re: 1 > 0 == True -> False Rotwang <sg552@hotmail.co.uk> - 2014-01-31 00:10 +0000
          Removal of iterable unpacking in function calls (was: 1 > 0 == True -> False) Ben Finney <ben+python@benfinney.id.au> - 2014-01-31 11:21 +1100
            Re: Removal of iterable unpacking in function calls Rotwang <sg552@hotmail.co.uk> - 2014-01-31 00:32 +0000
          Re: 1 > 0 == True -> False Joshua Landau <joshua@landau.ws> - 2014-01-31 00:32 +0000
      Re: 1 > 0 == True -> False Chris Angelico <rosuav@gmail.com> - 2014-01-31 11:01 +1100

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