generators and exceptions

[Greg Ewing (using news.cis.dfn.de)]

Andrew Bennetts wrote:
> But from which would you expect it to resume?  Just before the exception was
> raised?  Just after?  What about try/except blocks?  What if the exception
> was raised inside a function called by a generator?  What about an exception
> thrown by a generator that your generator calls?

It seems to me that what Mr. Evans wants is some way for
a generator to cause an exception to be raised in its
caller, without having to propagate the exception up
through its own stack frame first.

Suppose there were a hypothetical "yield raise" statement
which did this, then he could write

   def mygen(val):
     while val > 0:
       if val % 2:
         yield raise MyExc
       else:
         yield val
       val -= 1

The "yield raise" statement wouldn't disturb the control
flow of the generator at all -- it would be just like a
normal yield, except that whatever called the generator-
iterator's next() would get an exception instead of a
return value.

As for how *useful* such a thing would be, I don't know.
I won't be leaping up to write a PEP, though...

-- 
Greg Ewing, Computer Science Dept,
University of Canterbury,	
Christchurch, New Zealand
http://www.cosc.canterbury.ac.nz/~greg