How to pop the interpreter's stack?

[Carl Banks]

On Dec 25, 2:49 pm, Steven D'Aprano <steve
+comp.lang.pyt... at pearwood.info> wrote:
> On Sat, 25 Dec 2010 09:17:27 -0500, Robert Kern wrote:
> > On 12/24/10 5:14 PM, Ethan Furman wrote:
>
> >> There are also times when I change the exception being raised to match
> >> what python expects from that type of object -- for example, from
> >> WhatEverException to KeyError for a dict-like object. So in this regard
> >> I agree with Steven.
>
> > Steven isn't arguing that particular point here, nor is anyone arguing
> > against it.
>
> Emphasis on *here*.
>
> You will note that in Python 3, if you raise an exception inside an
> except block, both the *original* and the new exception are printed. This
> is great for fixing bugs inside except blocks, but terribly disruptive
> for catching one error and raising another error in it's place, e.g.:
>
> try:
>     x+0
> except ValueError, TypeError as e:
>     # x is not a numeric value, e.g. a string or a NAN.
>     raise MyError('x is not a number')
>
> The explicit raise is assumed to indicate a bug in the except block, and
> the original exception is printed as well.
>
> But that's a separate issue from what is being discussed here. What we're
> discussing here is the idea that a function should be able to delegate
> work to private subroutines without the caller being aware of that fact.
> When you return a value, the caller doesn't see the internal details of
> how you calculated the value, but if you deliberately raise an exception,
> the caller does. Often this is the right thing to do, but sometimes it
> isn't. E.g. you can't delegate input validation to a subroutine and raise
> inside the subroutine without obfuscating the traceback.


>>> import re
>>> re.compile(r"(")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/re.py", line 190, in compile
    return _compile(pattern, flags)
  File "/usr/lib/python2.6/re.py", line 245, in _compile
    raise error, v # invalid expression
sre_constants.error: unbalanced parenthesis


OHMYGOD HOW DARE the standard library allow the traceback list an
internal function that does input valididation!


Carl Banks