Jim Jewett wrote:
 > http://bugs.python.org/review/13897/diff/4186/14521
 > File Python/sysmodule.c (right):
 >
 > http://bugs.python.org/review/13897/diff/4186/14521#newcode211
 > Python/sysmodule.c:211: while ((exc_info->exc_type == NULL ||
 > exc_info->exc_type == Py_None) &&
 > This while loop is new, but it isn't clear how it is related to
 > encapsulating the exception state.  Is this fixing an unrelated bug, or
 > is it from generators, or ..?
 >
 > http://bugs.python.org/review/13897/show

Running generators form a stack, much like frames.
Calling a generator with next or send, pushes it onto the stack,
yielding pops it.

Now consider, if you will, the threadstate object as a sort of
non-yielding (it cannot be popped) generator which forms the base
of this stack.

In this patch, rather than swapping the exception state between 
generator-owned frame and threadstate whenever entering or leaving a 
generator, each generator (and the threadstate) has its own exception state.

It order to find the topmost exception state, sys.exc_info searches the 
stack of generators until it finds one.
In practice the generator stack will be very shallow, only 1 or 2 deep,
as it is rare to have generators calling other generators
(although this will become a bit more common with PEP 380).