Python

Quick Reference

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'