Respect isolated mode when patching sys.path

[karandhaodiyal28-hash]

Fixes #1916

Python's isolated mode (-I) must not prepend the script directory (for a file target) or the current directory (for -m / -c) to sys.path — that's the whole point of the flag, and it exists to prevent local modules from shadowing stdlib/third-party imports. debugpy re-implements Python's startup sys.path handling in src/debugpy/server/cli.py, but did so unconditionally, so a debuggee launched under -I still got the directory injected:

$ python -I -m debugpy --listen 5678 -c "import sys; print(repr(sys.path[0]))"
''                                          # cwd injected - isolated mode violated

$ python -I -c "import sys; print(repr(sys.path[0]))"
'/usr/lib/python3.11'                       # plain Python, correct

The comments in those three functions already say "like Python itself does for -m/-c" — but Python itself skips the insert under -I. This guards all three sites (run_file, run_module, run_code) with sys.flags.isolated, matching CPython semantics exactly. sys.flags.isolated has existed since 3.4, and non-isolated runs are completely unaffected.

The attach-to-PID injection path also inserts a directory, but it deletes it again right after importing its helper, so it's left as-is.

Testing

Added test_isolated_mode_sys_path (parametrized over file / -m / -c) to tests/debugpy/test_cli_args.py, asserting the target directory is not prepended to sys.path[0] when the debuggee runs under -I. The three new cases fail without this change and pass with it. Verified locally on Windows / Python 3.12; test_run.py shows no new failures introduced (identical pass/fail set with and without the change).

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[karandhaodiyal28-hash]

@microsoft-github-policy-service agree

[rchiodo]

🔒 Automated review in progress — @rchiodo is auto-reviewing this PR.

[rchiodo]

Warning · Non-blocking recommendation

CPython suppresses the sys.path[0] prepend whenever safe_path is set, not only under isolated. -P / PYTHONSAFEPATH=1 set safe_path without setting isolated, so python -P -m debugpy ... still injects the current directory. Please broaden the gate to account for sys.flags.safe_path (with a compatibility fallback for Python <3.11), or explicitly document why this fix intentionally covers only -I.

[verified]

[rchiodo]

Approved via Review Center.

[karandhaodiyal28-hash]

Thanks @rchiodo, that's a great catch — you're right that safe_path (via -P / PYTHONSAFEPATH) suppresses the prepend independently of isolated.

I've broadened the gate to cover both. All three sites (run_file, run_module, run_code) now go through a small helper:

def _skip_sys_path_prepend():
    # `sys.flags.safe_path` was added in Python 3.11; fall back to False on older versions.
    return sys.flags.isolated or getattr(sys.flags, "safe_path", False)

The getattr(..., False) keeps it safe on the pre-3.11 versions debugpy still supports. I also parametrized the regression test over both -I and -P (the -P cases are skipped below 3.11). Verified locally: with only the previous isolated gate the three -P cases fail, and they pass with this change; the full test_cli_args.py is green.

[rchiodo]

Warning · Non-blocking recommendation

Harden this subprocess-based regression test: add a timeout and preserve child stderr so a hung or failing debuggee produces a timely, diagnosable failure. The positional last-line parsing and negative-only assertion could also mask unrelated output or an incorrect replacement path; use an explicit marker and assert the expected path behavior. [verified]

[rchiodo]

Approved via Review Center.