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'