Python3 Stdlib Only

Constrained / Legacy Environments

Consult python3-core for standing defaults.

This skill is a LAST RESORT. Use ONLY when confirmed environment restrictions prevent dependency installation. Do not assume restrictions — verify with the user first.

flowchart TD
    Start([Need Python CLI script]) --> Q1{Environment allows network access<br>for dependency installation?}
    Q1 -->|Yes — standard environment| UseTyper[Use python3-cli with Typer+Rich<br>Less code, better UX, well-tested libraries]
    Q1 -->|No| Q2{Restriction type confirmed?}
    Q2 -->|No — assumption only| Stop[STOP — Verify with user first<br>Do not assume restrictions]
    Q2 -->|Yes — airgapped/no internet/no uv| StdLib[Use stdlib-only patterns]

Role

Create portable, dependency-free Python 3.11+ scripts that use only standard library at runtime. All code passes ruff check and the project type checker (ty or mypy / pyright per repo), with comprehensive pytest coverage.

Fundamental principle: Patterns are tools, not goals. Prefer simplicity, clarity, maintainability, and pragmatism over pattern correctness.

Dependency Policy

• Runtime: Standard library ONLY (no external packages)
• Dev-only: pytest, ruff, ty (preferred), mypy or pyright when matching the host project
• Runtime exceptions: Only if essential, with explicit written justification and pinning strategy

Required Behavior

• Preserve compatibility before applying modernization patterns
• Prefer stdlib solutions when dependency policy is restrictive
• Choose typing features valid for the project floor
• Make boundary wrappers explicit when Pydantic is unavailable
• Avoid recommending tools or syntax that exceed the project lane

Working Process

PHASE 0: Research

• Grep existing codebase for patterns to reuse
• List edge cases, integration points, and permission issues

PHASE 1: Refactor Sanity Check

• Measure lines, branches, redundancy, and readability
• Apply only changes that reduce duplication or improve clarity
• Walrus, DRY, SRP rules enforced with justification

Required report:

REFACTORING IMPACT ANALYSIS:
- Line count: X -> Y (∆ Z)
- Functions affected: N
- Patterns applied: [...]
- Complexity reduction: [...]
- Rejected changes: [reasons]

PHASE 2: Creation

• Confirm Python 3.11+ requirement
• Ensure no forbidden typing imports
• Design: argparse CLI, structured logging, error boundaries, config handling
• Async only for I/O-bound operations

PHASE 3: Validation

Automated:

python -m py_compile <file>
grep -E "from typing import .*(Dict|List|Set|Tuple|Optional|Union)\b" <file>
ruff check <file>
# Type check: match the host project — ty is default (`uv run ty check <file>`);
# use `mypy --strict <file>` when the project standardizes on mypy or uv is unavailable.

Checklist:

• Docstrings present
• CLI help works
• Unit tests for normal, edge, and error paths
• Cross-platform path handling
• Validation output shown

Required validation output:

VALIDATION REPORT:
✅ Syntax check: PASSED
✅ Forbidden patterns: NONE FOUND
✅ Native type hints: VERIFIED (X instances)
✅ Ruff: PASSED
✅ Type check (ty or mypy): PASSED
✅ Checklist items: COMPLETED

System Integration Patterns

Stdlib CLI

import argparse


def create_parser() -> argparse.ArgumentParser:
    parser = argparse.ArgumentParser(
        description="Tool description", formatter_class=argparse.RawDescriptionHelpFormatter
    )
    return parser

Stdlib Logging

import logging
from pathlib import Path


def setup_logging(level: str = "INFO", log_file: Path | None = None) -> None:
    handlers: list[logging.Handler] = [logging.StreamHandler()]
    if log_file:
        handlers.append(logging.FileHandler(log_file))
    logging.basicConfig(
        level=getattr(logging, level.upper(), logging.INFO),
        format="%(asctime)s %(name)s %(levelname)s %(message)s",
        handlers=handlers,
    )

Stdlib Config

import json, configparser, tomllib
from pathlib import Path
from typing import Any


def load_config(path: Path) -> dict[str, Any]:
    ext = path.suffix.lower()
    data = path.read_text()
    if ext == ".json":
        return json.loads(data)
    if ext == ".toml":
        return tomllib.loads(data)
    if ext in (".ini", ".cfg"):
        p = configparser.ConfigParser()
        p.read_string(data)
        return {sec: dict(p[sec]) for sec in p.sections()}
    raise ValueError(f"Unsupported config format: {ext}")

Privileged Command Execution

from shutil import which
import os, subprocess


def run_privileged_command(cmd: str, args: list[str]) -> subprocess.CompletedProcess[str]:
    if os.name != "posix":
        return subprocess.run([cmd, *args], capture_output=True, text=True)
    if os.geteuid() == 0:
        full = [cmd, *args]
    elif sudo := which("sudo"):
        full = [sudo, "-n", cmd, *args]
    else:
        full = [cmd, *args]
    return subprocess.run(full, capture_output=True, text=True, check=False)

Errors

import logging, sys


class ScriptError(Exception):
    """Base error for script-related failures."""


def handle_error(err: Exception, logger: logging.Logger) -> None:
    logger.error(f"Error: {err}")
    if logger.isEnabledFor(logging.DEBUG):
        logger.exception("Traceback")
    sys.exit(1)

Async

import asyncio
from collections.abc import Coroutine
from typing import Any


async def run_async(tasks: list[Coroutine[Any, Any, Any]]) -> list[Any]:
    return await asyncio.gather(*tasks, return_exceptions=True)

Multi-File Project Skeleton

When PEP 723 single-file approach is insufficient:

project_name/
├── main.py
├── core/
│   ├── __init__.py
│   ├── config.py
│   ├── logging_setup.py
│   └── exceptions.py
├── utils/
│   ├── __init__.py
│   └── helpers.py
├── tests/
│   ├── __init__.py
│   ├── test_main.py
│   └── test_core.py
├── config.example.toml
└── README.md

Code Quality Standards

• Pass ruff check and type checking clean (ty or mypy --strict per project)
• Google-style docstrings for modules, classes, and functions
• Use structural pattern matching, dataclasses, and walrus operator only if they reduce complexity and increase clarity
• Follow PEP 8

Testing Standards

Pytest with coverage of normal, edge, and error cases.

import json
from pathlib import Path
from module import load_config


def test_load_json(tmp_path: Path) -> None:
    path = tmp_path / "c.json"
    path.write_text(json.dumps({"k": "v"}))
    result = load_config(path)
    assert result == {"k": "v"}

Shebang

Stdlib-only scripts use #!/usr/bin/env python3 (no PEP 723 metadata — nothing to declare).

Validate with /python-engineering:shebangpython <file_path>.

Boundaries

• No deprecated syntax
• No platform-specific behavior without fallbacks
• No skipping validation or tests
• No delivery without a full Validation Report

References

• references/command-execution.md — timeout handling, privilege elevation, logging, type-safe command building
• references/type-safety-patterns.md — overloads, protocols, type narrowing, linter settings
• references/typing-strategy.md — Protocol vs TypeVar vs ParamSpec, abstract collections, Any boundaries, JSON handling