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Add 33 advanced pitfalls: asyncio, threading, interpreter internals, data systems - #2
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…a systems - docs/concurrency.md (20-30): asyncio task garbage collection, forgotten await, blocking the loop, swallowed cancellation, cross-thread loop calls, non-atomic +=, vanishing thread exceptions, Lock re-entry, threading.local under asyncio, fork() deadlocks, daemon threads. - docs/object-model.md (31-37): property vs __getattr__, lru_cache leaks, __eq__ removing __hash__, __exit__ swallowing, weakrefs to bound methods, mock.patch targets, double-imported __main__. - docs/interpreter.md (38-42): -O stripping asserts, class-body comprehension scope, locals() writes, id() reuse, int/str limit. - docs/data-systems.md (43-52): hash() sharding, NaN, nanosecond timestamps, float prices, DST arithmetic, fixed UTC offsets, utcnow(), struct alignment, non-standard JSON. - README: advanced contents table and an "In the wild" section on Home Assistant bug fixes and guard rails, each linked to the PR or source file. Examples 48 and 49 reproduce HA PRs #188537 and #188836. - One self-checking example per pitfall; all 52 run in CI. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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- examples/24: disable inherited asyncio debug mode before the stall demo. Under PYTHONASYNCIODEBUG=1 or -X dev, call_soon from a thread raised in the worker and the example hung forever. Waits now have a 5s timeout, so a regression fails instead of hanging. - docs/concurrency.md #21: -W error::RuntimeWarning does not fail on a never-awaited coroutine, because the warning fires during finalization and becomes an unraisable exception. Document the pytest filters that do fail (verified with pytest 9.2). - CI: also run every example under -X dev. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Summary
Adds Part II of the catalog: 33 advanced pitfalls on four new pages in
docs/, plus an "In the wild" section on how Home Assistant runs into and guards against them. Total goes from 19 to 52, every one with a self-checking example run in CI.New pages
call_soonfrom a thread stalls 500 ms ·threading.localgives Alice's request to Bob ·fork()child deadlocks on a copied lock'Device' object has no attribute 'temperature'·@lru_cachekeeps every instance alive ·mock.patchon the wrong modulepython -Odeletes validation · class-body comprehensions read the global ·id()reuse returning stale cache entries · 4300-digitint/strlimithash()sharding differs per process · floats lose nanoseconds ·datetimemerges trades 500 ns apart ·int(1.15 * 100) == 114· DST arithmetic ·structpadding ·NaNin JSONReal-world sources
block_async_io.py,verify_event_loop_thread(), bugbear rules).int64prices (10⁻⁹ units) and theUNDEF_PRICEsentinel.Determinism
Timing-sensitive pitfalls are forced to fail every time so CI can assert them:
gc.collect()for task collection,time.sleep(0)to widen the race window, a held lock acrossfork(), subprocesses for-O,PYTHONHASHSEEDand daemon threads. #29 and #50 skip on platforms withoutos.forkortime.tzset.Test plan
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