[hmr] Consider adding `-P` flag for CLI usage

[CNSeniorious000]

Background

Python 3.11 introduced -P flag to prevent unsafe paths in sys.path [What's New] [PEP 587] [CLI docs].

Official Definition: Prevents Python from automatically prepending potentially unsafe paths to sys.path.

Enable via:

• -P command-line flag [Python docs]
• PYTHONSAFEPATH=1 environment variable [env var]
• sys.flags.safe_path (runtime check) [sys.flags]

Behavior [Python docs]:

• python script.py: doesn't prepend script's directory
• python -m module: doesn't prepend current working directory
• python -c code and python (REPL): doesn't prepend empty string (current directory)

Design Goals

From official proposals [PEP 587] and discussions:

1. Security: Prevents malicious local modules from being accidentally imported
2. Path injection protection: Stops local directories from shadowing stdlib packages [CPython #95754] [Better errors PR #113769]
3. Consistency: Aligns interactive environments with command-line behavior

Historical Context

GitHub issue #57684 tracked this feature request:

• Original proposal: --mainpath/--nomainpath command-line options
• Evolution: -P flag and PYTHONSAFEENV (later renamed PYTHONSAFEPATH) [BPO #13475]
• Core maintainers: Victor Stinner et al.
• Implementation: [initconfig.c] [PyConfig.safe_path]

Industry Adoption

• IPython 9.7: Added PYTHONSAFEPATH support [changelog] [PR #15014]
• pdb: Respects safe_path behavior [PR #111762]
• IPython aligns with Python's native security without requiring separate --ignore_cwd configuration

Proposal

Add -P/--safe-path flag and PYTHONSAFEPATH support to our Python execution environment. This aligns with Python standards and improves security—critical for web-based Python consoles handling untrusted code.

Why This Matters

Python's official security enhancement prevents dependency path attacks in multi-user or untrusted environments. As an online IDE, implementing this feature would protect users from module shadowing vulnerabilities.

[linear]

PROM-78