def verifyFileExists(self, fileDir, fileName):
        """
        Helper which verifies that C{fileName} exists in C{fileDir} and it has
        some content.

        @param fileDir: A path to a directory.
        @type fileDir: L{FilePath}

        @param fileName: The last path segment of a file which may exist within
            C{fileDir}.
        @type fileName: L{str}

        @raise: L{FailTest <twisted.trial.unittest.FailTest>} if
            C{fileDir.child(fileName)}:

                1. Does not exist.

                2. Is empty.

                3. In the case where it's a path to a C{.html} file, the
                   content looks like an HTML file.

        @return: L{None}
        """
        # check that file exists
        fpath = fileDir.child(fileName)
        self.assertTrue(fpath.exists())

        # check that the output files have some content
        fcontents = fpath.getContent()
        self.assertTrue(len(fcontents) > 0)

        # check that the html files are at least html-ish
        # this is not a terribly rigorous check
        if fpath.path.endswith('.html'):
            self.assertIn(b"<body", fcontents)

        Helper which verifies that C{fileName} exists in C{fileDir} and it has
        some content.

        @param fileDir: A path to a directory.
        @type fileDir: L{FilePath}

        @param fileName: The last path segment of a file which may exist within
            C{fileDir}.
        @type fileName: L{str}

        @raise: L{FailTest <twisted.trial.unittest.FailTest>} if
            C{fileDir.child(fileName)}:

                1. Does not exist.

                2. Is empty.

                3. In the case where it's a path to a C{.html} file, the
                   content looks like an HTML file.

        @return: L{None}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/test/test_release.py