[pypy-dev] Re: deep breakage

[Guido van Rossum]

> From: Armin Rigo <arigo at tunes.org>
> To: Michael Hudson <mwh at python.net>
> Subject: Re: [pypy-dev] [gmane.comp.python.pypy] deep breakage
> 
> Hello Michael,
> 
> On Sat, Jun 07, 2003 at 08:38:32PM +0100, Michael Hudson wrote:
> > try:
> >     "x" + 1
> > except TypeError, e:
> >     raise e
> > 
> > because here 'e' gets bound to a *string* and then this is what's
> > reraised.

This is of course false.  e gets a TypeError instance (or at least
that's what it does in CPython, and what it should do).

> This is the same problem as in CPython. Most failing internal
> operations call PyErr_SetString(), which causes a string to be used
> as the exception value.

No, as the argument to the exception constructor.

> The hack is that when the user code reaches an 'except' clauses the
> exception is "normalized", which translates the pair "TypeError
> class + string" into the pair "TypeError class + TypeError
> instance".

Right, and for all practical purposes,

  raise TypeError, "foo"

is the same as

  raise TypeError("foo")

> We might either choose to mimic this behavior or clean it up a bit,
> e.g. by always using internally a single object as the exception
> instance.

That's fine.  CPython uses the pair so that for certain exceptions
raised *internally* (by C code, not by raise statements) the
instantiation is never done if the exception is caught and cleared by
C code further down the stack.  For example, when iterating over a
built-in sequence, the StopIteration exception is raised but never
instantiated.

> String exceptions are being deprecated, but still we could
> reasonably emulate them, say by inventing a StringException
> class. This would isolate the hacky parts around this specific class
> (like setting sys.exc_type to something else than
> current_exception.__class__ for this particular case).

I would not even bother.  String exceptions are strongly deprecated
part of the language.

--Guido van Rossum (home page: http://www.python.org/~guido/)