run-external --redirect-stderr is confusing / there's no way to operate on stderr?

[gandalf3]

Describe the bug

the documentation here says

--redirect-stderr, -: redirect stderr to the pipeline

which lead me to believe this would work

>  run-external --redirect-stderr "python" "-c" "import sys; print('hello on stderr', file=sys.stderr)" | inspect
Error: nu::shell::pipeline_mismatch

  × Pipeline empty.
   ╭─[entry #1:1:1]
 1 │  run-external --redirect-stderr "python" "-c" "import sys; print('hello on stderr', file=sys.stderr)" | inspect
   ·                                                                                                         ───┬───
   ·                                                                                                            ╰── no input value was piped in
   ╰────

if im understanding right, --redirect stderr does not mean "redirect stderr to the pipeline" (what i assumed to mean "the next command's stdin"), it currently means "redirect stderr to the stderr stream in the pipeline", which is ignored by most (all?) nu commands, and as far as i can tell from #4396, exists purely for use with capture or redirects.

trying to redirect stderr to stdout as follows kind of works, but rather than going into nu's pipeline as expected, it is written to the terminal.

> run-external --redirect-stderr "python" "-c" "import sys; print('hello on stderr', file=sys.stderr)" err> /dev/stdout | inspect
hello on stderr
Error: nu::shell::pipeline_mismatch

  × Pipeline empty.
   ╭─[entry #27:1:1]
 1 │ run-external --redirect-stderr "python" "-c" "import sys; print('hello on stderr', file=sys.stderr)" err> /dev/stdout | inspect
   ·                                                                                                                         ───┬───
   ·                                                                                                                            ╰── no input value was piped in
   ╰────

How to reproduce

run-external --redirect-stderr "python" "-c" "import sys; print('hello on stderr', file=sys.stderr)" | inspect

Expected behavior

I expected nu to have an easy way to read and operate on stderr, and in general manipulate io streams freely, since it is common functionality in posix/bash/zsh etc.

Screenshots

No response

Configuration

key	value
version	0.86.0
branch	makepkg
commit_hash	5d8763e
build_os	linux-x86_64
build_target	x86_64-unknown-linux-gnu
rust_version	rustc 1.73.0 (cc66ad468 2023-10-03) (Arch Linux rust 1:1.73.0-1)
cargo_version	cargo 1.73.0
build_time	2023-10-18 08:45:09 +00:00
build_rust_channel	release
allocator	mimalloc
features	dataframe, default, extra, sqlite, trash, which, zip
installed_plugins

Additional context

No response

[MarikaChlebowska]

I agree, I think redirect-stderr should simply move stderr of the command to stdout so next command can use it as an input. I'm not sure if we should keep stderr of the run-external empty or keep it as it is right now. If someone from the maintainers gives me a green light I think I can change it.