Interpreter Methods For Main Interpreter Fail Unexpectedly

Bug report

Bug description:

The following should run without error:

import contextlib
from concurrent import interpreters

assert __name__ == '__main__'

DIV1 = 'v' * 40
DIV2 = '^' * 40

@contextlib.contextmanager
def dump_traceback():
    try:
        yield
    except Exception:
        import traceback
        print()
        print(DIV1)
        traceback.print_exc()
        print(DIV2)

# First try with the current interpreter.
interp0 = interpreters.get_current()
assert interp0 is interpreters.get_main()

# Interpreters "running main" cannot exec or call.
assert interp0.is_running
with dump_traceback():
    try:
        interp0.exec('None')
    except interpreters.InterpreterError as exc:
        if 'running main' not in str(exc):
            raise
with dump_traceback():
    try:
        interp0.call(print, 'spam!', flush=True)
    except interpreters.InterpreterError as exc:
        if 'running main' not in str(exc):
            raise

# prepare_main() should still work
assert 'spam' not in globals()
with dump_traceback():
    interp0.prepare_main(spam=11)
    assert spam == 11

# Try with a subinterpreter
interp1 = interpreters.create()

assert 'q' not in globals()
with dump_traceback():
    interp1.exec("""if True:
        from concurrent import interpreters
        q = interpreters.create_queue()
        interp0 = interpreters.get_main()
        interp0.prepare_main(q=q)
        q.put_nowait(42)
        """)
    assert 'q' in globals()
    val = q.pop()
    assert val == 42

Instead, the four different calls against interp0 raise InterpreterError with "cannot ... unrecognized interpreter 0". The calls to exec() and call() should fail with something like "running main". The call to prepare_main() should work. Whether prepare_main() should work or not is a separate matter. Regardless, the main interpreter should be recognized.

Note that interp0.is_running does work, as expected.

CPython versions tested on:

3.15, 3.14

Operating systems tested on:

No response

Linked PRs

• gh-139521