How to pop the interpreter's stack?

[Robert Kern]

On 12/24/10 4:24 AM, Steven D'Aprano wrote:
> On Thu, 23 Dec 2010 22:38:05 -0800, Carl Banks wrote:

>> OTOH, going the extra mile to hide useful information from a user is
>> asinine. As a user, I will decide for myself how I want to use
>> implementation-defined information, and I don't want the implementor to
>> decide this for me. It's bad enough if an implementor fails to provide
>> information out of laziness, but when they deliberately do extra work to
>> hide information, that's self-importance and arrogance.
>
> But that of course is nonsense, because as the user you don't decide
> anything of the sort. The developer responsible for writing the function
> decides what information he provides you, starting with whether you get
> an exception at all, where it comes from, the type of exception, and the
> error message (if any).

Carl isn't arguing that the user is or should be responsible for this sort of 
thing. He is arguing that developers should be responsible for doing this in 
such a way that is beneficial for the developer/user down the road.

> Once this information has been passed on to you,
> you're free to do anything you like with it, but you never get to choose
> what information you get -- I'm not suggesting any change there. All I'm
> suggesting is that there should be a way of reducing the boilerplate
> needed for this idiom:
>
> def _validate_arg(x):
>      if x == 'bad input': return False
>      return True
>
> def f(arg):
>      if not _validate_arg(arg):
>          raise ValueError
>      process(arg)
>
> to something more natural that doesn't needlessly expose implementation
> details that are completely irrelevant to the caller.

Except that the *caller* never gets the traceback (unless if it deliberately 
inspects the stack for some metaprogramming reason). It gets the exception, and 
that is the same no matter what you do. The developer/user gets the traceback, 
and those implementation details *are* often important to them.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco