gh-143874: Fix pdb expression result output stream in _exec_in_closure()

[hyongtao-code]

Fixes an issue where results of expressions executed via _exec_in_closure() were written to sys.stdout instead of the debugger output stream. Adds a regression test to ensure expression results are consistently emitted via pdb.stdout.

• Issue: pdb: some expression results are printed to the debuggee stdout #143874

[cocolato]

I think the current test case doesn't actually test the remote debug mode. We need add some tests in test_remote_pdb.py:

cpython/Lib/test/test_remote_pdb.py

Lines 832 to 833 in c461aa9

	class RemotePdbTestCase(unittest.TestCase):
	"""Tests for the _PdbServer class."""

[cocolato]

Let's add some tests like this:

class RemotePdbTestCase(unittest.TestCase):

    ...

    def test_remote_closure_exec(self):
        self.pdb._exec_in_closure('(lambda: 123)()', {}, {})
        outputs = self.sockfile.get_output()
        messages = [o['message'].strip() for o in outputs if 'message' in o]
        self.assertIn("123", messages)

[hyongtao-code]

I think the current test case doesn't actually test the remote debug mode.

Thank you very much — I couldn’t agree more with your feedback.

Before that, I spent a long time trying to find an appropriate place for the corresponding test case, and still didn’t get it right. <(_ _)>

[gaogaotiantian]

The issue is real but it's not because print prints to sys.stdout. It's because remote pdb overrides self.message so we have to use that. sys.stdout is actually replaced with self.stdout in local pdb mode.

[hyongtao-code]

but it's not because

Totally agree. The actual execution path is:

self.message()
 → local pdb: print(..., file=self.stdout)
 → remote pdb: send over the socket

Thank you for helping identify the RCA and clarifying the underlying cause–and–effect relationship.