Python 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit 
(Intel)] on win32
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>>> def coroutine():
...     try:
...             raise Exception('bla')
...     except:
...             yield 'foo'
...             raise
...
>>> for _ in coroutine():
...     pass
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in coroutine
TypeError: exceptions must be classes or instances, not NoneType

Seems that yielding in the except clause makes python forget the current 
exception, so a bar "raise" statement raises None, instead of the 
original exception.
changing except to except Exception,e: and raise to raise e throws the 
original exception, but then the original stack trace is lost.

This is fixed in Python 3. I don't know if it's worth backporting the
changes because it may be hard to avoid also backporting the
incompatibilities in the exception model.

This won't get backported now that 2.7 is in maintenance.