[Python-Dev] Chained Exceptions

[Guido van Rossum]

[Brett C.]
> If a new exception is raised (e.g., not a bare 'raise') while a current
> exception is active (e.g., sys.exc_info() would return something other than a
> tuple of None), then the new exception is made the active exception and the now
> old exception is assigned to the new exception's context attribute to be the
> old exception.

Define "raise". Does that involve a raise statement? What about 1/0?
What if you call a method that executes 1/0? What if that method
catches that exception? What about the StopIteration conceptually
raised by next() called by the for-loop implementation? (Often it
doesn't get instantiated at all when the next() method belongs to a
built-in iterator.)

I believe there are (at least) two use cases:

(1) I catch some low-level exception (e.g. socket.error) and turn it
into a high-level exception (e.g. an HTTPRequestFailed exception).

(2) I write some exception handling code and somehow a bug in the
handler (or an uncooperative environment, e.g. a full disk) causes the
exception handling code to trip over an exception.

I'm fairly certain (but not 100%) that Ping meant to include both use cases.

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