An alternative way to delay the file opening, is to return an instance that has a `filename` attribute, and has an `open` method.  This can be compactly added to the `FileContext` that I defined in the previous patch.  The `FileContext` provides the `_ostest` function (testing using os.access), and wrapper for stdin/out.


    class FileOpener(argparse.FileContext):
        # delayed FileType; alt to use of partial()
        # sample use:
        # with args.input.open() as f: f.read()
        def __call__(self, string):
            string = self._ostest(string)
            self.filename = string
            return self
        def open(self):
            return self.__delay_call__(self.filename)()
        file =  property(open, None, None, 'open file property')