`PYTHONSTARTUP` causing traceback source mismatch in interactive console

[CNSeniorious000]

Bug report

Bug description:

When PYTHONSTARTUP environment variable is set, the startup file execution permanently leaves a __loader__ attribute in the __main__ module's __dict__. This causes linecache to use __loader__.get_source() for source code retrieval, resulting in tracebacks in the interactive console incorrectly showing source lines from the PYTHONSTARTUP file instead of the actual executed code.

The issue occurs in Modules/main.c in pymain_run_startup():

1. pymain_run_startup() calls _PyRun_SimpleFileObject(fp, startup, 0, &cf)
2. _PyRun_SimpleFileObject() in Python/pythonrun.c sets __loader__ via set_main_loader(dict, filename, "SourceFileLoader")
3. The cleanup code only removes __file__ and __cached__ but not __loader__:
   

   cpython/Python/pythonrun.c

   Lines 533 to 543 in 04c4628

   	done:
   	if (set_file_name) {
   	if (PyDict_PopString(dict, "__file__", NULL) < 0) {
   	PyErr_Print();
   	}
   	if (PyDict_PopString(dict, "__cached__", NULL) < 0) {
   	PyErr_Print();
   	}
   	}
   	Py_XDECREF(main_module);
   	return ret;

CPython versions tested on:

3.13

Operating systems tested on:

Windows (It seems this issue only appears on Windows)

[picnixz]

I guess this is similar to #129098, right?

[CNSeniorious000]

Yeah. Now every PYTHONSTARTUP file needs to set __loader__ = None by itself to prevent this issue.

[picnixz]

Hum, apparently there was a previous attempt of a fix in #137625 and #137576. Isn't it then already fixed? (3.13.8 is not yet released so that might be the reason why it's not working "yet"). The commit is actually just after the 3.13.7 release.

[CNSeniorious000]

You're right, sorry for missing those. Thanks for the context!