Bug report

Bug description:

According to the documentation, all PYTHON* environment variables are ignored when -I is passed as a command-line option to Python.

Let's make the asyncio REPL comply with that by checking sys.flags.isolated before PYTHONSTARTUP.

#140288 rearranges and solidifies startup tests for any REPLs. It will help in testing this. Let's merge that PR first, and I'll submit a patch for this one shortly after.

Reproduction:

$ echo 'print("you should not be seeing this!")' > t.py $ PYTHONSTARTUP=t.py ./python -Im asyncio asyncio REPL 3.15.0a1+ (heads/main:06c779474c7, Oct 27 2025, 02:33:08) [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. you should not be seeing this! >>> import asyncio >>>

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux