Python lru_cache pitfalls - eviction demo, unhashable list TypeError, methods keep instances alive, stale values, shared mutable results
Python f-string formatting - 2.675 formats as 2.67, round-half-even in .0f, thousands and percent specs, = debug, 3.10 quote and backslash limits
Python collections gotchas - defaultdict read creates keys, Counter drops negative counts, deque maxlen discards, shallow dict copy shares nested
Python random reproducibility - random.seed and private Random instances, shuffle returns None, sample vs choices, secrets for tokens
JavaScript Promise patterns - all vs allSettled vs race vs any results, forEach async trap, timeout wrapper limits, concurrency pool of 3
Node fs error codes decoded - ENOENT EEXIST ENOTEMPTY EISDIR, mkdir recursive, rm force, wx exclusive create, atomic rename, invalid utf8 reads
Node --env-file loading - quotes and comments parsing, shell variables override the file, process.env values are strings, validated config module
Node worker_threads for CPU work - blocked event loop gives 0 ticks, 4 parallel workers in 1.4x single time, worker error events, when not to use threads
Pydantic models as LLM tool schemas - model_json_schema output, inline $defs refs, extra=forbid, error types to return, lax vs strict coercion
SQLite maintenance - online backup API, VACUUM shrinking 2180 to 1092 KB, WAL and shm files, wal_checkpoint(truncate), integrity_check
