`code.InteractiveConsole` leaks to stderr when stdin is non-interactive and `-i` isn't specified

[johnslavik]

Bug report

Bug description:

sys._baserepl() doesn't write to stdout when stdin is not a TTY:

❯ ./python <<< ''

(As clearly resembled in the code:)

cpython/Python/pythonrun.c

Lines 74 to 82 in 5f91d5d

	if (_Py_FdIsInteractive(fp, filename)) {
	res = _PyRun_InteractiveLoopObject(fp, filename, flags);
	if (closeit) {
	fclose(fp);
	}
	}
	else {
	res = _PyRun_SimpleFileObject(fp, filename, closeit, flags);
	}

We can force it to write to stdout with the -i flag.

❯ ./python -i <<< ''
Python 3.15.0a0 (heads/main:5f91d5d9a41, Oct 12 2025, 22:21:59) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> >>> 

It's a mechanism currently used in tests:

cpython/Lib/test/test_repl.py

Lines 30 to 38 in 5f91d5d

	def spawn_repl(*args, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, **kw):
	"""Run the Python REPL with the given arguments.
	kw is extra keyword args to pass to subprocess.Popen. Returns a Popen
	object.
	"""
	# To run the REPL without using a terminal, spawn python with the command
	# line option '-i' and the process name set to '<stdin>'.

However, the code console family doesn't exhibit the same behavior:

❯ ./python -m code <<< ''
Python 3.15.0a0 (heads/main:5f91d5d9a41, Oct 12 2025, 22:21:59) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> >>> 
now exiting InteractiveConsole...

❯ ./python -m asyncio <<< ''
asyncio REPL 3.15.0a0 (heads/main:5f91d5d9a41, Oct 12 2025, 22:21:59) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Use "await" directly instead of "asyncio.run()".
Type "help", "copyright", "credits" or "license" for more information.
>>> import asyncio
>>> >>> 
exiting asyncio REPL...

The output that we're seeing here is gibberish.

The reason for that is that both of these interactive interpreters use input() for prompts.
input() writes to

1. stdout, if stdin is a TTY or stdout is redirected
2. stderr, if stdin is not a TTY or stderr is redirected

(The relationship of input() with stdout and stderr was widely discussed in #46221, but this issue doesn't particularly criticize that behavior).

Unless some existing pipelines rely on this existing behavior (which I doubt), perhaps we'd like to align code and friends to the original behavior and make them work with -i flag too (which will be a separate issue, preferable completed first, before this one).

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

[johnslavik]

Whoops, this is invalid. I misunderstood -i initially. Specifying -i doesn't affect how standard I/O is done, it changes whether the REPL is run in the first place. I misjudged the outputs -- the REPL always writes to stdout, no matter what. -i has nothing to do with it.