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This looks like an oversight. In the implementation in [bltinmodule.c][1], if checking sys.stdout.fileno() fails, it clears the error without setting tty = 0. It's the [same in 3.5][2].
 /* We should only use (GNU) readline if Python's sys.stdin and
 sys.stdout are the same as C's stdin and stdout, because we
 need to pass it those. */
 tmp = _PyObject_CallMethodId(fin, &PyId_fileno, "");
 if (tmp == NULL) {
 PyErr_Clear();
 tty = 0;
 }
 else {
 fd = PyLong_AsLong(tmp);
 Py_DECREF(tmp);
 if (fd < 0 && PyErr_Occurred())
 return NULL;
 tty = fd == fileno(stdin) && isatty(fd);
 }
 if (tty) {
 tmp = _PyObject_CallMethodId(fout, &PyId_fileno, "");
 if (tmp == NULL)
 PyErr_Clear();
 else {
 fd = PyLong_AsLong(tmp);
 Py_DECREF(tmp);
 if (fd < 0 && PyErr_Occurred())
 return NULL;
 tty = fd == fileno(stdout) && isatty(fd);
 }
 }
[1]: https://hg.python.org/cpython/file/b4cbecbc0781/Python/bltinmodule.c#l1709
[2]: https://hg.python.org/cpython/file/7a088af5615b/Python/bltinmodule.c#l1839