Re: raise None

From	Terry Reedy <tjreedy@udel.edu>
Newsgroups	comp.lang.python
Subject	Re: raise None
Date	2015-12-30 23:00 -0500
Message-ID	<mailman.99.1451534427.11925.python-list@python.org> (permalink)
References	<56847239$0$1590$c3e8da3$5496439d@news.astraweb.com>

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On 12/30/2015 7:09 PM, Steven D'Aprano wrote:
> I have a lot of functions that perform the same argument checking each time:
>
> def spam(a, b):
>      if condition(a) or condition(b): raise TypeError
>      if other_condition(a) or something_else(b): raise ValueError
>      if whatever(a): raise SomethingError
>      ...
>
> def eggs(a, b):
>      if condition(a) or condition(b): raise TypeError
>      if other_condition(a) or something_else(b): raise ValueError
>      if whatever(a): raise SomethingError
>      ...
>
>
> Since the code is repeated, I naturally pull it out into a function:
>
> def _validate(a, b):
>      if condition(a) or condition(b): raise TypeError
>      if other_condition(a) or something_else(b): raise ValueError
>      if whatever(a): raise SomethingError
>
> def spam(a, b):
>      _validate(a, b)
>      ...
>
> def eggs(a, b):
>      _validate(a, b)
>      ...
>
>
> But when the argument checking fails, the traceback shows the error
> occurring in _validate, not eggs or spam. (Naturally, since that is where
> the exception is raised.) That makes the traceback more confusing than it
> need be.
>
> So I can change the raise to return in the _validate function:
>
> def _validate(a, b):
>      if condition(a) or condition(b): return TypeError
>      if other_condition(a) or something_else(b): return ValueError
>      if whatever(a): return SomethingError
>
>
> and then write spam and eggs like this:
>
> def spam(a, b):
>      ex = _validate(a, b)
>      if ex is not None: raise ex
>      ...
>
>
> It's not much of a gain though. I save an irrelevant level in the traceback,
> but only at the cost of an extra line of code everywhere I call the
> argument checking function.

It is nicer than the similar standard idiom

try:
     _validate(a, b)
except Exception as e:
     raise e from None

If you could compute a reduced traceback, by copying one, then this 
might work (based on Chris' idea.

def _validate(a, b):
     ex = None
     if condition(a) or condition(b): ex = TypeError
     elif other_condition(a) or something_else(b): ex = ValueError
     elif whatever(a): ex = SomethingError
     if ex:
         try: 1/0
         except ZeroDivisionError as err: tb = err.__traceback__
         tb = <copy of tb with this frame omitted>
         raise ex.with_traceback(tb)

> But suppose we allowed "raise None" to do nothing. Then I could rename
> _validate to _if_error and write this:
>
> def spam(a, b):
>      raise _if_error(a, b)
>      ...
>
>
> and have the benefits of "Don't Repeat Yourself" without the unnecessary,
> and misleading, extra level in the traceback.
>
> Obviously this doesn't work now, since raise None is an error, but if it did
> work, what do you think?

Perhaps a bit too magical, but maybe not.

-- 
Terry Jan Reedy

Thread

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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