Given:

@contextmanager
def gen():
    print '__enter__'

    try:
        yield
    finally:
        print '__exit__'

with gen():
    raise StopIteration('body')


running the above results in:

__enter__
__exit__


I would have expected instead

__enter__
__exit__
Traceback (most recent call last):
  File "test25.py", line 53, in <module>
    raise StopIteration('body')
StopIteration: body


Note that doing:

with gen():
    raise GeneratorExit('body')

does the expected thing:

__enter__
__exit__
Traceback (most recent call last):
  File "test25.py", line 53, in <module>
    raise GeneratorExit('body')
GeneratorExit: body