Run Python Safely

Run Python Safely

Execute Python code safely by performing AST-based static analysis to detect potentially dangerous operations before execution.

MANDATORY USAGE

Agents MUST use this skill when executing any Python code they have generated.

This is a CRITICAL RULE for all agents in this repository. Before running Python code via Bash:

1. Invoke this skill using the Skill tool
2. Pass the code or file path as the argument
3. Only proceed if the code passes safety checks

Exceptions (when you can skip this skill):

• Running tests via uv run pytest
• Running validation via uv run .claude/scripts/validate_code.py
• Running standard CLI commands (e.g., ruff format, ty check)
• User-provided scripts that have already been reviewed

When to Use

Use this skill when you need to run Python code that you've generated. The skill analyzes the code for dangerous patterns and either:

• Executes the code if no issues are found
• Blocks execution and provides feedback if dangerous operations are detected

Usage

# Execute inline code
uv run .claude/scripts/run_python_safely.py -c "print(2 + 2)"

# Execute code from file
uv run .claude/scripts/run_python_safely.py -f script.py

# Execute with custom timeout (in seconds)
uv run .claude/scripts/run_python_safely.py -t 60 -c "print('done')"

Exit Codes

Output Format

Safe Code (Executed)

[EXECUTED]
<stdout from executed code>

Unsafe Code (Blocked)

[BLOCKED] Code execution blocked due to safety concerns:
  - Import: os (file system and process operations)
  - Builtin: eval (arbitrary code execution)

If this code is safe, ask the user for permission to run directly.

Blocked Operations

Imports

The following imports are blocked because they can access the file system, network, or execute arbitrary code:

• os, sys, subprocess - System and process operations
• shutil - High-level file operations
• socket, requests, httpx, urllib, ftplib - Network operations
• pickle, shelve, marshal - Serialization (arbitrary code execution risk)
• ctypes - C library access
• multiprocessing, threading - Process/thread spawning
• importlib, builtins - Dynamic import/builtin access

Builtin Functions

These builtin functions are blocked:

• eval, exec, compile - Arbitrary code execution
• open - File system access
• __import__ - Dynamic module import
• getattr, setattr, delattr - Dynamic attribute access
• globals, locals, vars - Namespace access
• breakpoint - Debugger invocation
• input - User input during execution

Methods

These method names are blocked on any object (may have rare false positives):

• write_text, write_bytes, touch - File creation/writing
• mkdir, rmdir, unlink, rmtree - Directory/file deletion
• rename, replace - File/directory moving
• symlink_to, hardlink_to, link_to - Link creation
• chmod, lchmod - Permission modification

When Code is Blocked

If the skill blocks code that you believe is safe:

1. Explain to the user what the code does
2. Ask the user for explicit permission to run it directly
3. If granted, use python -c "code" or python script.py directly

Safe Operations

The following are always allowed:

• Standard library modules: math, json, re, datetime, collections, itertools, functools, pathlib (read operations), etc.
• All safe builtins: print, len, str, int, list, dict, range, enumerate, zip, map, filter, etc.
• Reading files via pathlib: Path.read_text(), Path.read_bytes(), Path.exists(), Path.is_file(), etc.
• Pure computation and data manipulation

Intentionally Allowed Modules

The following modules are intentionally NOT blocked:

tempfile

Allows agents to create temporary working files for intermediate results. The tempfile module creates files in system temp directories that are automatically cleaned up. This is useful for:

• Storing intermediate computation results
• Creating scratch files during data processing
• Writing temporary config files for subprocesses

Note: While tempfile can create files, these are confined to temp directories and don't pose the same risk as arbitrary filesystem access via os or shutil.

asyncio

Allows agents to use async/await patterns for concurrent operations. While asyncio can spawn concurrent tasks, it doesn't directly cause filesystem damage or network access (those would still be blocked by their respective module checks).

Examples

Safe Code Examples

# Math computation
import math
print(math.sqrt(16))

# JSON processing
import json
data = {"key": "value"}
print(json.dumps(data))

# List comprehension
squares = [x**2 for x in range(10)]
print(squares)

Blocked Code Examples

# BLOCKED: os import
import os
os.system("ls")

# BLOCKED: open builtin
with open("file.txt", "w") as f:
    f.write("data")

# BLOCKED: unlink method
from pathlib import Path
Path("file.txt").unlink()

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