[issue2833] __exit__ silences the active exception

[Antoine Pitrou]

Antoine Pitrou <pitrou at free.fr> added the comment:

As Amaury said, lexically nested exception handlers make re-raising
behaviour buggy. In Py3k, a workaround is to instead write:

   try:
      raise Exception("foo")
   except Exception as :
      try: raise KeyError("caught")
      except KeyError: pass
      raise e

With the slight inconvenience that the "raise e" line will be appended
to the original traceback.

If we want bare "raise" statements to work as expected after a nested
exception handler, we'll need to add proper exception stacking, for
example by adding the exception value as a member of PyTryBlock. The
effect on performance should also be measured.

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<http://bugs.python.org/issue2833>
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