# Python

> Write reliable Python avoiding mutable defaults, import traps, and common runtime surprises.

- Skill: `johnalbertini14-glitch/python` (Agent Skill, multi-file: 9 files)
- Install (CLI): `npx skillmds@latest add johnalbertini14-glitch/python`
- Raw SKILL.md: https://api.skillmd.com/api/skills/johnalbertini14-glitch/python/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: johnalbertini14-glitch (https://skillmd.com/u/johnalbertini14-glitch)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/johnalbertini14-glitch/python

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## Quick Reference

| Topic | File |
|-------|------|
| Dynamic typing, type hints, duck typing | `types.md` |
| List/dict/set gotchas, comprehensions | `collections.md` |
| Args/kwargs, closures, decorators, generators | `functions.md` |
| Inheritance, descriptors, metaclasses | `classes.md` |
| GIL, threading, asyncio, multiprocessing | `concurrency.md` |
| Circular imports, packages, __init__.py | `imports.md` |
| Pytest, mocking, fixtures | `testing.md` |

## Critical Rules

- `def f(items=[])` shares list across all calls — use `items=None` then `items = items or []`
- `is` checks identity, `==` checks equality — `"a" * 100 is "a" * 100` may be False
- Modifying list while iterating skips elements — iterate over copy: `for x in list(items):`
- GIL prevents true parallel Python threads — use multiprocessing for CPU-bound
- Bare `except:` catches `SystemExit` and `KeyboardInterrupt` — use `except Exception:`
- `UnboundLocalError` when assigning to outer scope variable — use `nonlocal` or `global`
- `open()` without context manager leaks handles — always use `with open():`
- Circular imports fail silently or partially — import inside function to break cycle
- `0.1 + 0.2 != 0.3` — floating point, use `decimal.Decimal` for money
- Generator exhausted after one iteration — can't reuse, recreate or use `itertools.tee`
- Class attributes with mutables shared across instances — define in `__init__` instead
- `__init__` is not constructor — `__new__` creates instance, `__init__` initializes
- Default encoding is platform-dependent — always specify `encoding='utf-8'`