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Re: raise None

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On Thu, Dec 31, 2015 at 12:26 PM, Steven D'Aprano <steve@pearwood.info> wrote:
> Traceback (most recent call last):
>   File "spam", line 19, in this
>   File "spam", line 29, in that
>   File "spam", line 39, in other
> ThingyError: ...
>
>
> I think this is a win for debuggability. (Is that a word?) But it's a bit
> annoying to do it today, since you have to save the return result and
> explicitly compare it to None. If "raise None" was a no-op, it would feel
> more natural to just say raise _validate() and trust that if _validate
> falls out the end and returns None, the raise will be a no-op.

(Yes, it is.)

Gotcha. So here's an alternative possibility. Instead of raising None
doing nothing, what you really want is to have _validate signal an
error with one less level of traceback - that is, you want it to raise
an exception from the calling function.

class remove_traceback_level:
    def __enter__(self): return self
    def __exit__(self, type, value, traceback):
        if type is None: return
        tb = traceback
        while tb.tb_next: tb = tb.tb_next
        tb.tb_next = None
        raise value from traceback

def _validate(a, b):
    with remove_traceback_level():
        if condition(a) or condition(b): raise TypeError
        if other_condition(a) or something_else(b): raise ValueError
        if whatever(a): raise SomethingError


The trouble is that this doesn't actually work, because tb_next is
read-only. But is there something along these lines that would make a
function raise exceptions as if it were in another function?

ChrisA

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raise None Steven D'Aprano <steve@pearwood.info> - 2015-12-31 11:09 +1100
  Re: raise None Paul Rubin <no.email@nospam.invalid> - 2015-12-30 16:19 -0800
  Validation in Python (was: raise None) Ben Finney <ben+python@benfinney.id.au> - 2015-12-31 11:26 +1100
  Re: raise None Chris Angelico <rosuav@gmail.com> - 2015-12-31 11:38 +1100
    Re: raise None Steven D'Aprano <steve@pearwood.info> - 2015-12-31 12:26 +1100
      Re: raise None Ben Finney <ben+python@benfinney.id.au> - 2015-12-31 12:44 +1100
        Re: raise None Steven D'Aprano <steve@pearwood.info> - 2015-12-31 15:07 +1100
          Re: raise None Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2015-12-31 12:19 +0000
            Re: raise None Steven D'Aprano <steve@pearwood.info> - 2016-01-01 02:35 +1100
              Re: raise None Chris Angelico <rosuav@gmail.com> - 2016-01-01 02:53 +1100
              Re: raise None Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2015-12-31 16:46 +0000
                Re: raise None Steven D'Aprano <steve@pearwood.info> - 2016-01-01 04:50 +1100
              Re: raise None "Martin A. Brown" <martin@linux-ip.net> - 2015-12-31 09:30 -0800
              Re: raise None Ben Finney <ben+python@benfinney.id.au> - 2016-01-01 07:18 +1100
                Re: raise None Johannes Bauer <dfnsonfsduifb@gmx.de> - 2016-01-02 12:47 +0100
              Re: raise None Chris Angelico <rosuav@gmail.com> - 2016-01-01 09:48 +1100
                Re: raise None Steven D'Aprano <steve@pearwood.info> - 2016-01-04 16:19 +1100
                Re: raise None Dan Sommers <dan@tombstonezero.net> - 2016-01-04 06:09 +0000
                Re: raise None Rustom Mody <rustompmody@gmail.com> - 2016-01-03 22:39 -0800
              Re: raise None Ben Finney <ben+python@benfinney.id.au> - 2016-01-01 10:27 +1100
                Re: raise None Marko Rauhamaa <marko@pacujo.net> - 2016-01-01 02:29 +0200
                Re: raise None Steven D'Aprano <steve@pearwood.info> - 2016-01-04 16:19 +1100
                Re: raise None Rustom Mody <rustompmody@gmail.com> - 2016-01-03 21:53 -0800
                Re: raise None Rustom Mody <rustompmody@gmail.com> - 2016-01-04 03:55 -0800
                Re: raise None Rustom Mody <rustompmody@gmail.com> - 2016-01-03 21:53 -0800
              Re: raise None Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-12-31 23:36 +0000
              Re: raise None Chris Angelico <rosuav@gmail.com> - 2016-01-01 10:39 +1100
              Re: raise None Chris Angelico <rosuav@gmail.com> - 2016-01-01 10:41 +1100
              Re: raise None Rustom Mody <rustompmody@gmail.com> - 2016-01-03 19:04 -0800
                Re: raise None Chris Angelico <rosuav@gmail.com> - 2016-01-04 14:31 +1100
                Re: raise None Steven D'Aprano <steve@pearwood.info> - 2016-01-04 14:48 +1100
                Re: raise None Chris Angelico <rosuav@gmail.com> - 2016-01-04 14:56 +1100
                Re: raise None Rustom Mody <rustompmody@gmail.com> - 2016-01-03 20:46 -0800
              Re: raise None Christian Gollwitzer <auriocus@gmx.de> - 2016-01-04 08:28 +0100
      Re: raise None Chris Angelico <rosuav@gmail.com> - 2015-12-31 13:12 +1100
      Re: raise None Cameron Simpson <cs@zip.com.au> - 2015-12-31 15:03 +1100
        Re: raise None Steven D'Aprano <steve@pearwood.info> - 2015-12-31 16:12 +1100
          Re: raise None Cameron Simpson <cs@zip.com.au> - 2015-12-31 16:45 +1100
  Re: raise None Terry Reedy <tjreedy@udel.edu> - 2015-12-30 23:00 -0500
  Re: raise None Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-12-31 15:58 +0000
  Re: raise None Johannes Bauer <dfnsonfsduifb@gmx.de> - 2015-12-31 17:17 +0100

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