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There's an obscure bug in sys.exc_info after a yield statement.
 def test():
 def raising_generator():
 try:
 raise IndexError("inner exception")
 except IndexError:
 yield 3
 # Here, sys.exc_info() ought to refer to the inner
 # exception, but instead it refers to the outer one.
 try:
 raise ValueError("outer exception")
 except ValueError:
 for i in raising_generator(): pass
sys.exc_info gets reset even if there's no outer exception.
The attached (failing) patch highlights this problem, and tests other
sys.exc_info behavior around function calls.