[Python-Dev] Re: PEP-317

[Greg Ewing]

I'm worried about the proposed elimination of deferred instantiation
of exceptions in Python code. The PEP asserts that this is an
optimisation that should only be of interest in C code.

But in Pyrex, the distinction between Python and C is
blurred. Currently, if you raise an exception in Pyrex using the
implicit instantiation syntax, instantiation is also deferred, and I
think this is a good thing.  But if the implicit instantiation syntax
goes away, this will no longer be possible while keeping the same
syntax for the raise statement in Python and Pyrex.

It seems to me it's not necessary to eliminate implicit instantiation
altogether, but only to remove the ambiguity between implicit and
explicit instantiation. This could be done by mandating that

   raise expr

*never* instantiates implicitly, whereas

   raise classexpr, valueexpr

*always* instantiates implicitly, possibly also deferred.

Specification of a traceback would be done with a new keyword, e.g.

   raise expr [,expr] traceback tb

which I think is a good idea anyway, since it makes it a lot clearer
what the currently-somewhat-obscure third argument actually is.

Greg Ewing, Computer Science Dept, +--------------------------------------+
University of Canterbury,	   | A citizen of NewZealandCorp, a	  |
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