Name: Python
Author: ivangdavila

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'