with foo as bar:
    baz()

# The fundamental, simple concept behind with is that
# it's equivalent to this:
bar = foo.__enter__()
try:
    baz()
finally:
    foo.__exit__(None, None, None)

# But there are less common cases where you don't just
# want to clean up on exit, you want to do different cleanup
# depending on whether you were exiting by exception or
# normally. To make those cases work, with has an inner
# try/except, to have somewhere to capture that information:
bar = foo.__enter__()
exc = (None, None, None)
try:
    try:
        baz()
    except:
        exc = sys.exc_info()
        raise
finally:
    foo.__exit__(*exc)

# Finally, there are even less common cases where you
# want the context manager to be able to examine the exc
# info and decide whether to swallow it or propagate it.
# To make those cases work, you need a second __exit__
# call in the inner block, at which point we're
# effectively equivalent to what's specified in PEP 343:
bar = foo.__enter__()
no_exception = True
try:
    try:
        baz()
    except:
        no_exception = False
        if not foo.__exit__(*sys.exc_info()):
            raise
finally:
    if no_exception:
        foo.__exit__(None, None, None)