I wouldn’t expect to see a message for stderr because there is nowhere else to send the message. However with my patch for Issue 5319, at least we get a non-zero exit status.

Brandon: I agree this should be under sys.stdin/out/err; I wasn’t saying to put it under the io.TextIOBase documentation. What I meant was one option would be to refer to that class as a duck-typing API. Say something like: “sys.stdout [etc] may be replaced with other objects, as long as they implement [the write-only parts, etc of] the TextIOBase API [optionally ‘buffer’ etc]”. Exact details to be determined.