Build a Python argparse CLI - subparsers, choices, count and append flags, exit code 2, why options after a subcommand fail

Fix Pydantic v2 ValidationError - read loc and type, Optional needs default, strict vs lax, v1 to v2 renames, validators, BaseSettings move

Pytest mocking - patch where the name is used, autospec to catch bad calls, side_effect for retries, monkeypatch, freezegun

Find and fix flaky pytest tests - random order, repeat and xdist reproduction, shared state, fixture scope, time, network and async races

Pytest fixtures explained - session/module/function order logged, autouse, module-scope state leak failing assert 2 == 1, --setup-show, fixture not found

Diagnose slow PostgreSQL queries - EXPLAIN ANALYZE, bad estimates, locks and safe index creation

Fix Prisma Migrate errors - drift, shadow database P3006, failed migration P3009 resolve, baselining an existing DB, dev vs deploy, directUrl

OpenSSL for local TLS - create a self-signed cert with SAN, read subject and expiry, openssl verify -CAfile, SHA-256 fingerprints

Fix pandas SettingWithCopyWarning - use .loc for updates, .copy() after filtering, chained assignment and Copy-on-Write in pandas 3

Tune pgvector for fast, accurate search - cosine and L2 operator classes, HNSW vs IVFFlat, ef_search, filtered queries and recall measurement

Python packaging basics - pyproject.toml, src layout, pip install -e, console_scripts entry point, packages.find where error

Profile slow Python - cProfile ranking, timeit min-of-N, list vs set membership measured, join vs +=, tracemalloc and generators

Python file I/O - explicit encoding, UnicodeDecodeError, newline translation, utf-8-sig for BOM CSVs, mkdir exist_ok, os.replace atomic writes

Fix Python multiprocessing - PicklingError for lambdas, local function hangs Pool.map under spawn, locks not picklable, shared Value counter

Fix Python JSONDecodeError Expecting value - print status and body, HTML and empty responses, BOM, NDJSON, truncated bodies, retry rules

Fix Python ImportError - circular imports, local file shadowing stdlib, ModuleNotFoundError, python -m for packages

Fix Python logging - basicConfig no-op, duplicate lines from propagation, lazy formatting, logger.exception, JSON logs

Python collections gotchas - defaultdict read creates keys, Counter drops negative counts, deque maxlen discards, shallow dict copy shares nested

Python context managers run live - @contextmanager needs try/finally, __exit__ returning True swallows errors, suppress, ExitStack

Python lru_cache pitfalls - eviction demo, unhashable list TypeError, methods keep instances alive, stale values, shared mutable results