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Re: cascading python executions only if return code is 0

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On 22Dec2013 16:53, Roy Smith <roy@panix.com> wrote:
> In article <mailman.4512.1387748397.18130.python-list@python.org>,
>  Cameron Simpson <cs@zip.com.au> wrote:
> > Roy's code _depends_ upon the return value being equivalent to False.
> 
> Yes.  You view this as a flaw.  I view it as a feature :-)

When I write functions which return a boolean indicating success/failure,
I try to make that boolean be "true" for success.

Now, I do take the point that these functions seem to take the
unix-exit-code convention that zero is success (leaving the many
values of "non-zero" to indicate flavours of failure as desired,
as we have many types of exceptions).

Or, possibly, that a non-zero return indicates the number of errors
encountered; I do that myself for things like option or file parsing,
where I explicitly want to parse of much of a command line (or
whatever) as possible before rejecting things; few things annoy me
as much as a command that barfs about the first usage error and
aborts instead of barfing multiple times and aborting. Unless it
is a command that does the same and then fails to recite a usage
message after the barf. (Yes, almost every GNU command on the planet:
I'm looking at you!)

However, in this count-of-errors scenario I tend to try to return
a list of errors, not a count.

But regardless, I consider code that goes:

  a() or b() or c()

as a test for _success_ of a(), b() and c() in succession to be
misleading at best. When I write that above incantation it is a
test for failure: try this, or try that, or finally try this other
thing.

> > A better approach would be:
> > 
> >   a() == 0 and b() == 1 and c() == 0
> > 
> > i.e. to explicitly check each return code against the expected/desired
> > value instead of relying on some flukey coincidental property of
> > the (arbitrary) numeric value returned.
> 
> You're assuming it's arbitrary.  I'm saying do it that way by design.  

The counter example the above is based upon deliberately returned
1 for success from b(), IIRC. Different design.

The OP was unclear about his/her design rationale.

> > Better still is the suggestion elsewhere in the thread to make the functions
> > raise exceptions on error instead of returning a number.
> 
> Possibly.  But, I think of exceptions as indicating that something went 
> wrong.

I think of failure as "something went wrong".
Yes, I'll grant there are shades of intent here.

> There's two possible things the OP was trying to do here:
> 
> 1) He intends that all of the functions get run, but each one can only 
> get run if all the ones before it succeed.  In that case, I agree that 
> the exception pattern makes sense.

His cascading if-statement in the OP suggested this to me.

> 2) He intends that each of the functions gets tried, and the first one 
> that can return a value wins.  If that's the case, the "or" chaining 
> seems more natural.

I'm pretty sure that wasn't his intent, again based on my recollection
of the OP. But I still dislike "a() or b() or c()" as a test for
chained success; I think it is a bad idiom.

Cheers,
-- 
Cameron Simpson <cs@zip.com.au>

I must construct my own System, or be enslaved to another Man's.
        - William Blake

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cascading python executions only if return code is 0 Frank Cui <ycui@outlook.com> - 2013-12-22 15:37 -0300
  Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-22 14:17 -0500
    Re: cascading python executions only if return code is 0 Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-12-22 19:31 +0000
      Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-22 15:05 -0500
        Re: cascading python executions only if return code is 0 Cameron Simpson <cs@zip.com.au> - 2013-12-23 08:39 +1100
          Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-22 16:53 -0500
            Re: cascading python executions only if return code is 0 Cameron Simpson <cs@zip.com.au> - 2013-12-23 09:11 +1100
            Re: cascading python executions only if return code is 0 Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-12-22 22:10 -0500
            Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-23 14:22 +1100
            Re: cascading python executions only if return code is 0 Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-12-23 03:43 +0000
            Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-23 14:45 +1100
            Re: cascading python executions only if return code is 0 Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-12-23 03:54 +0000
            Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-23 14:59 +1100
    RE: cascading python executions only if return code is 0 Frank Cui <ycui@outlook.com> - 2013-12-22 16:10 -0300
    Re: cascading python executions only if return code is 0 Ned Batchelder <ned@nedbatchelder.com> - 2013-12-22 14:49 -0500
      Re: cascading python executions only if return code is 0 Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-12-23 00:08 +0000
    RE: cascading python executions only if return code is 0 Frank Cui <ycui@outlook.com> - 2013-12-22 16:35 -0300
      Re: cascading python executions only if return code is 0 Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2013-12-23 13:30 +1300
  Re: cascading python executions only if return code is 0 Rick Johnson <rantingrickjohnson@gmail.com> - 2013-12-22 14:27 -0800
    RE: cascading python executions only if return code is 0 Frank Cui <ycui@outlook.com> - 2013-12-22 19:14 -0300
    Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-23 09:51 +1100
    Re: cascading python executions only if return code is 0 Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-12-22 23:02 +0000
      Re: cascading python executions only if return code is 0 Rick Johnson <rantingrickjohnson@gmail.com> - 2013-12-22 15:57 -0800
        Re: cascading python executions only if return code is 0 Ned Batchelder <ned@nedbatchelder.com> - 2013-12-23 07:27 -0500
        Re: cascading python executions only if return code is 0 Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-12-23 12:44 +0000
  Re: cascading python executions only if return code is 0 Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-12-23 00:25 +0000
    Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-22 20:24 -0500
      Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-23 12:35 +1100
      Re: cascading python executions only if return code is 0 Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-12-23 13:33 +1100
        Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-23 14:09 +1100
        Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-22 23:57 -0500
          Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-23 16:09 +1100
          Re: cascading python executions only if return code is 0 Ethan Furman <ethan@stoneleaf.us> - 2013-12-23 04:45 -0800
            Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-23 10:10 -0500
              Re: cascading python executions only if return code is 0 Ethan Furman <ethan@stoneleaf.us> - 2013-12-23 08:00 -0800
                Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-26 20:37 -0500
                Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-27 12:44 +1100
                Re: cascading python executions only if return code is 0 Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-12-26 21:20 -0500
                Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-27 13:27 +1100
                Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-26 23:29 -0500
                Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-27 15:40 +1100
                Re: cascading python executions only if return code is 0 Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-12-27 11:15 -0500
                Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-27 11:42 -0500
                Re: cascading python executions only if return code is 0 Rustom Mody <rustompmody@gmail.com> - 2013-12-27 22:39 +0530
                Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-28 08:24 +1100
                Re: cascading python executions only if return code is 0 pecore@pascolo.net - 2013-12-28 00:59 +0100
                Re: cascading python executions only if return code is 0 Gene Heskett <gheskett@wdtv.com> - 2013-12-27 19:12 -0500
                Re: cascading python executions only if return code is 0 Tim Chase <python.list@tim.thechases.com> - 2013-12-26 20:50 -0600
                Re: cascading python executions only if return code is 0 Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-12-27 15:39 +1100
                Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-27 15:45 +1100
                Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-27 00:05 -0500
                Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-27 16:15 +1100
                Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-27 00:41 -0500
                Re: cascading python executions only if return code is 0 Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-12-27 18:00 +0000
                Re: cascading python executions only if return code is 0 Grant Edwards <invalid@invalid.invalid> - 2013-12-30 17:36 +0000
              Re: cascading python executions only if return code is 0 Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-12-24 04:05 +1100
                Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-23 11:03 -0800
                Re: cascading python executions only if return code is 0 Chris Angelico <rosuav@gmail.com> - 2013-12-24 06:12 +1100
                RE: cascading python executions only if return code is 0 Nick Cash <nick.cash@npcinternational.com> - 2013-12-23 19:31 +0000
                Re: cascading python executions only if return code is 0 Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-12-24 14:41 +1100
                Re: cascading python executions only if return code is 0 Roy Smith <roy@panix.com> - 2013-12-23 22:58 -0500
              Re: cascading python executions only if return code is 0 Peter Otten <__peter__@web.de> - 2013-12-23 18:45 +0100
          Re: cascading python executions only if return code is 0 Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-12-23 13:20 +0000
        Re: cascading python executions only if return code is 0 Ethan Furman <ethan@stoneleaf.us> - 2013-12-23 04:38 -0800
        Re: cascading python executions only if return code is 0 Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-12-23 10:12 -0500
          Re: cascading python executions only if return code is 0 Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-12-24 04:13 +1100
        Re: cascading python executions only if return code is 0 Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-12-23 15:22 +0000

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