[Python-Dev] Another traceback idea [was: except Exception as err, tb] (original) (raw)

Greg Ewing greg.ewing at canterbury.ac.nz
Fri Mar 2 23:00:53 CET 2007

Nick Maclaren wrote:

The instance contains all of the information about the details, such as the exact operation, the values and the context (including the traceback). It CAN'T be an object, because it is not 'assignable' (i.e. a value) - it is inherently bound to its context. You can turn it into an object by copying its context into an assignable form, but the actual instance is not assignable.

This has given me another idea:

Instead of instantiating the exception when it's raised, what about instantiating it when it's caught instead?

Suppose for a moment that we take what might seem to be a retrograde step, and go back to the old way of raising exceptions, where you supply the type and arguments separately:

raise MyException, ("Bogus value:", x)

Now instead of immediately "normalising" this by creating the exception, we keep all three parts (type, args, traceback) separate while we search for a handler.

If we find one of the form

except MyException as e:

then at that point we instantiate the exception, attach the traceback and assign it to e.

However, if we find one of the form

except MyException:

then we don't need to instantiate it at all! This would address the issue of efficiently using exceptions for flow control, since for that kind of use you're probably not going to want the exception object, in which case your except clauses will be of the latter form.

Now, I'm not proposing that the raise statement should actually have the above syntax -- that really would be a step backwards. Instead it would be required to have one of the following forms:

raise ExceptionClass

or

raise ExceptionClass(args)

plus optional 'with traceback' clauses in both cases. However, the apparent instantiation call wouldn't be made -- it's just part of the syntax.

-- Greg

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