hello_oneline.py

(lambda flask: 
    (lambda Flask: 
        (lambda app: 
            (lambda hello: 
                [f() for f in [
                    lambda : app.route("/")(hello), 
                    lambda : app.run() if __name__ == "__main__" else None
                    ]
                ][-1]
            )(lambda : 
                "Hello World"
              )
        )(Flask
            (
                __name__
            )
        )
    )(getattr
        (flask, "Flask")
    )
)(
    __import__("flask")
)