def __exit__(self, exc_type=None, exc_value=None, traceback=None):
if self.old_value is not None:<EOL><INDENT>os.environ[UPDATE_VARIABLE] = self.old_value<EOL><DEDENT>elif UPDATE_VARIABLE in os.environ:<EOL><INDENT>del os.environ[UPDATE_VARIABLE]<EOL><DEDENT>
Restore the previous value of :data:`UPDATE_VARIABLE` when leaving the context.