The proper procedure is to raise the new exception inside of the __exit__
handler.
You should not raise the exception that was passed in though; to allow for context manager chaining, in that case you should just return a falsey value from the handler. Raising your own exceptions is however perfectly fine.
Note that it is better to use the identity test is
to verify the type of the passed-in exception:
def __exit__(self, ex_type, ex_val, tb):
if ex_type is VagueThirdPartyError:
if ex_val.args[0] == 'foobar':
raise SpecificException('Foobarred!')
# Not raising a new exception, but surpressing the current one:
if ex_val.args[0] == 'eggs-and-ham':
# ignore this exception
return True
if ex_val.args[0] == 'baz':
# re-raise this exception
return False
# No else required, the function exits and `None` is returned
You could also use issubclass(ex_type, VagueThirdPartyError)
to allow for subclasses of the specific exception.