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OpenSSL for local TLS - create a self-signed cert with SAN, read subject and expiry, openssl verify -CAfile, SHA-256 fingerprints
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Read SQLite EXPLAIN QUERY PLAN - SCAN vs SEARCH, composite index column order, temp B-tree for ORDER BY, and queries that ignore indexes
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Python file I/O - explicit encoding, UnicodeDecodeError, newline translation, utf-8-sig for BOM CSVs, mkdir exist_ok, os.replace atomic writes
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Why Node exits - unhandled rejections, async throws, setTimeout errors escaping try/catch, EADDRINUSE, emitter error events, heap OOM
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Pytest mocking - patch where the name is used, autospec to catch bad calls, side_effect for retries, monkeypatch, freezegun
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Use Python requests safely - always set timeout, raise_for_status, catch RequestException, reuse Session, check JSON content type
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Use Python sqlite3 safely - ? parameter binding vs injection, implicit transactions, with conn commits but never closes, thread rule
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Python concurrency - ThreadPoolExecutor vs ProcessPoolExecutor measured: CPU work slower with threads, I/O faster, lost exceptions
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Build a Python argparse CLI - subparsers, choices, count and append flags, exit code 2, why options after a subcommand fail
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Python environment variable config - bool(false-string) trap, KeyError vs default, empty vs unset, subprocess env replace, .env parsing
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Process huge files in Node - readline for await, stream/promises pipeline, backpressure, unhandled stream errors that crash
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NDJSON and JSON Lines in Python and jq - write, stream, skip corrupt lines, Extra data errors, NaN, ensure_ascii
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Read mypy errors - Optional None narrowing, return-value and arg-type mismatches, attr-defined, strict no-untyped-def, implicit Optional
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Fix .gitignore not working - git rm --cached for tracked files, check-ignore -v, negation inside ignored dirs, secrets remain in history
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Verify webhook HMAC-SHA256 signatures - raw body not re-serialized JSON, timestamp tolerance, compare_digest and timingSafeEqual
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URL encoding cheat sheet - Python quote vs urlencode, JS encodeURIComponent vs URLSearchParams, + vs %20, parse_qs, base64url
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Fix pandas merge surprises - duplicate keys multiply rows, validate one_to_one, indicator audits, int vs object key errors, NaN keys match
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Fix pandas SettingWithCopyWarning - use .loc for updates, .copy() after filtering, chained assignment and Copy-on-Write in pandas 3
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Fix pandas groupby and resample - NaN keys dropped, dropna=False, count vs size, transform shape, empty resample periods
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NumPy gotchas run live - broadcasting shapes, int8 overflow wraparound, slices as views, NaN poisoning, isclose for floats
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Python gotchas run live - late-binding lambdas return [2, 2, 2], exhausted generators, dict mutation RuntimeError, shared nested lists
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Python sorting gotchas - numeric strings, key=int, case and accent order, stable multi-key sorts, None TypeError, NaN breaking order
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JavaScript numbers lose precision - 0.1+0.2, MAX_SAFE_INTEGER, JSON.parse rounding big IDs, BigInt, toFixed and money handling
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JavaScript gotchas run live - default sort as text, map(parseInt), == coercion, Date parsing UTC vs local, NaN includes, var closures
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