[Python-Dev] Idea for avoiding exception masking

[Guido van Rossum]

> When working on the sets module, a bug was found
> where trapping an exception (a TypeError for a mutable
> argument passed to a dictionary) resulted in masking
> other errors that should have been passed through
> (potential TypeErrors in the called iterator for example).
> 
> Now, Walter is working on a bug for map(), zip(), and
> reduce() where errors in the getiter() call are being
> trapped, reported as TypeError (for non-iterability),
> but potentially masking other real errors in 
> a __iter__ routine.  The current proposed solution is
> to remove the PyErr_Format call so that the underlying
> error message gets propagated up its original form.
> The downside of that approach is that it loses information
> about which argument caused the error.
> 
> So, here's the bright idea.  Add a function, 
> PyErr_FormatAppend, that leaves the original message
> intact but allows additional information to be added
> (like the name of the called function, identification
> of which argument triggered the error, a clue as
> to how many iterations had passed, or anything else
> that makes the traceback more informative).
> 
> Python code has a number of cases where a higher
> level routine traps an exception and reraises it with
> new information and losing the lower level error
> detail in the process.

Was a conclusion reached in this thread?  It seems a good idea.

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