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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _app(self):
 """The application object to work with; this is either the app that we have been bound to, or the current application. """

        if self.app is not None:
            return self.app

        ctx = _request_ctx_stack.top
        if ctx is not None:
            return ctx.app

        try:
            from flask import _app_ctx_stack
            app_ctx = _app_ctx_stack.top
            if app_ctx is not None:
                return app_ctx.app
        except ImportError:
            pass

        raise RuntimeError('assets instance not bound to an application, '+
                            'and no application in current context')