`SystemExit` raised by `PYTHONSTARTUP` can prevent the REPL from starting

[johnslavik]

Bug report

Bug description:

To reproduce:

1. The new REPL

❯ PYTHONSTARTUP=<(echo 'raise SystemExit') ./python
Python 3.15.0a3+ (heads/main:5b5263648f2, Dec 20 2025, 07:19:41) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.

2. The basic REPL

❯ PYTHON_BASIC_REPL=1 PYTHONSTARTUP=<(echo 'raise SystemExit') ./python
Python 3.15.0a3+ (heads/main:5b5263648f2, Dec 20 2025, 07:19:41) [GCC 15.2.1 20250808 (Red Hat 15.2.1-1)] on linux
Type "help", "copyright", "credits" or "license" for more information.

(both actually hit the same code path -- Py_RunMain -> pymain_run_python -> pymain_run_stdin -> pymain_run_startup -> _PyRun_SimpleFileObject -> PyErr_Print -> PyErr_PrintEx -> _PyErr_PrintEx -> handle_system_exit)

While this is largely an edge case of the regular REPLs, copying this semantics just feels wrong in Python code.
I've run into this while adding PYTHONSTARTUP exception handling to the asyncio REPL in GH-140288.

Should I be keeping asyncio REPL in parity with Python-side REPLs behavior in this regard?
Do we want to fix this in the C code?

And lastly, as part of this issue we would document the exception handling of PYTHONSTARTUP.

@ZeroIntensity stdlib interpreter-core docs topic-repl 3.13 3.14 3.15

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

[ZeroIntensity]

stdlib interpreter-core docs topic-repl 3.13 3.14 3.15

We try not to mix stdlib, interpreter-core, and docs. Let's pick one.

[johnslavik]

stdlib interpreter-core docs topic-repl 3.13 3.14 3.15

We try not to mix stdlib, interpreter-core, and docs. Let's pick one.

Actually... extension-modules. 😅
This is essentially about pymain_run_startup.

[johnslavik]

I'm very unsure about the likely user expectation here, but I'm leaning toward thinking that the REPL should always open despite any PYTHONSTARTUP failures.

[johnslavik]

@ZeroIntensity topic-repl also