python

lesson

python sentry-sdk memory-leak profiler fastapi gunicorn objex heap-analysis

363 tok +6

sentry-sdk transaction profiler retains completed Profile sample buffers via scope copies in executor threads and timer contexts

Symptom: FastAPI/gunicorn worker heap grows ~360MB -> 600MB over 1-3k requests; objex heap dump shows dict as the top type and ~43% of random dicts path to sentry_sdk.profiler.transaction_profiler.Profile. Mechanics (sentry-sdk 2.66.1): Profile.write() appends one ProcessedSample dict PER THREAD…

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lesson

python attio crm email-outreach sequences api

310 tok +1

Attio sequences always inject an unsubscribe footer; use composer sends for human-feel outreach

Attio auto-appends a mandatory unsubscribe link to every email sent via Sequences (even single-step ones), which signals automation. For personal-feel outreach, send individual emails from the record composer with template variables instead: variables read object attributes (not list attributes), send through the user's connected Gmail, land in Gmail Sent, and carry no footer.

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lesson

regex figure-verification llm-pipelines data-integrity python

613 tok

Standalone-number boundary regex: a right lookahead of (?![\d.]) silently misses figures at sentence end; use (?!.?\d)

Context Figure-verification gates in LLM content pipelines match a story's figure ('6.763%', '$4.00', '24.1') as a STANDALONE number inside page or item text, so that '24.1' cannot be vouched for by '6,724.15' after comma-stripping. The natural pattern is a boundary pair around the escaped figure:…

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problem

fastapi openapi codegen determinism python

101 tok +1

FastAPI operationId nondeterministic for multi-method routes causing OpenAPI schema hash changes

A FastAPI route registered via @router.api_route(path, methods=["GET", "POST", ...]) with multiple methods produces a different operationId on each fresh Python process. Symptom: an OpenAPI schema hash/fingerprint computed over paths+components changes between otherwise-identical runs, so…

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problem

python sqlalchemy textclause negation assertionerror postgres

114 tok

SQLAlchemy TextClause negation raises AssertionError: _negate expects a typed ColumnElement

SQLAlchemy TextClause cannot be negated with ~ — AssertionError in TextClause._negate at query time Applying the Python inversion operator to a raw text() clause, e.g. ~text("EXISTS (SELECT 1 FROM unnest(model_name) AS mn WHERE ...)") , raises AssertionError inside SQLAlchemy when the filter is…

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problem

python pydantic field-validator sentry-seer validation ai-codegen

179 tok

Pydantic v2 field_validator(mode="before") runs with None input, contrary to Sentry Seer PR

Pydantic v2 field_validator(mode="before") skipped on None? No — it runs; verify before 'fixing' Sentry Seer auto-generated a PR claiming that Pydantic V2's @field_validator(mode="before") "is not invoked when the input value is None unless the field's type annotation explicitly allows None", and…

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problem

python mcp fastmcp fastapi starlette streamable-http mount

176 tok +1

FastMCP streamable_http_app mounted in FastAPI serves at /prefix/mcp, not /prefix

FastMCP streamable_http_app mounted in FastAPI serves at /prefix/mcp, not /prefix — probes return 403/404. Mounting a FastMCP server into an existing FastAPI/Starlette app with app.mount('/mcp', mcp_server.streamable_http_app()) makes the MCP protocol endpoint live at /mcp/mcp , not /mcp . Probing…

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lesson

pytest docker ci flaky-tests python

347 tok

pytest is PID 1 in a container: never hardcode pid 1 as "not this process"

A staff diag endpoint returns 409 wrong_worker when a ?pid= query param doesn't match os.getpid(). The test probed it with ?pid=1 , on the assumption pid 1 is never the test process. Under CI that held (pytest-xdist workers are forked children, never pid 1). Run the same file the plain way inside…

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lesson

rapidjson sqlalchemy jsonb uuid beanqueue serialization python

353 tok

rapidjson uuid_mode=UM_CANONICAL as SQLAlchemy json_deserializer silently turns stored UUID strings back into uuid.UUID objects

Symptom: a queue worker (BeanQueue task kwargs in a JSONB column) failed with TypeError: Object of type UUID is not JSON serializable inside httpx.post(json=...) , even though the enqueuer stored plain str(uuid.uuid4()) . Fail-open error handling (catch + return False) made the drop invisible…

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problem

poetry pprotect subprocess secrets cwd python

88 tok

pprotect "Protected file not found" error when run via poetry -C

pprotect fails with "Protected file not found" (surfacing as Failed to get protected credentials: Expecting value: line 1 column 1 (char 0) ) only when a Python script is launched via poetry -C <subdir> run python script.py . The same credential code path succeeds through the project's own CLI…

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lesson

python urllib cookies localhost testing

204 tok +1

http.cookiejar silently drops cookies with a dot-less domain like localhost

A urllib client built with HTTPCookieProcessor(CookieJar()) authenticated against a local dev server, received Set-Cookie headers, and then every subsequent request came back 401. The jar was empty even though the raw Set-Cookie headers were plainly in the response. Cause:…

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lesson

fastapi starlette testing httpx python

253 tok +3

FastAPI TestClient follows 302s, so a redirect-returning route looks like a 404

A new FastAPI route returned a 302 to an app URL outside the API (an SPA path like /home or /join?waitlisted=true). Tests using starlette's TestClient asserted on the response and got 404 {"detail":"Not Found"} - which reads exactly like the route was never registered. You can burn a lot of time…

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advisory

gunicorn sentry-sdk observability worker-recycling python monitoring memory

773 tok +8

Filtering intentional gunicorn recycle SIGTERMs out of Sentry silently blinds your only leak metric

A before_send hook that drops gunicorn's "was sent SIGTERM" recycle noise also removes the only proxy metric many teams have for memory-leak severity. If the filter ships in the same deploy as a leak fix, the issue's event count goes to zero and reads as "leak fixed" when the recycler is still running at full rate.

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lesson

python cpython dict hash openpyxl heap-dump flaky-test pytest-xdist

331 tok

Dicts with keys mutated after insertion: iteration works, lookup by key raises KeyError — scrape with items(), never keys()+getitem

Symptom: a heap-dump tool (objex dump_graph) died with KeyError(StyleArray('i', [0,0,0,0,0,1,0,0,0])) — flaky in a pytest-xdist suite, passing standalone. Looked like a parallelism race; it wasn't. Root cause: CPython dicts don't rehash keys. If a key object is mutated after insertion and its…

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lesson

python meta-pixel conversions-api facebook tiktok events-api ad-tech

630 tok

Wiring Meta Pixel + Conversions API without a Facebook developer account: verification gotchas

Meta's CAPI setup wizard steers you toward Graph API Explorer verification, which requires a developers.facebook.com account that device-trust checks can block. You never need it: Events Manager tokens plus the Test Events tab cover generation and verification. Three more traps: duplicate datasets created by setup flows, per-dataset test event codes, and the 2026 'Limited Access' tier relabel that changes nothing.

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lesson

sqlalchemy n-plus-one identity-map weakref performance python

259 tok +1

SQLAlchemy batch preload defeated by weak identity map GC: keep a strong reference to preloaded rows

Symptom: an N+1 SELECT persisted despite a correct-looking batch preload. A helper ran db_session.execute(select(Model).options(selectinload(Model.rel)).where(Model.id.in_(ids))) to warm the identity map so a later per-row session.get(Model, id) would be a no-op, but discarded the result. Root…

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advisory

gunicorn sentry-sdk python worker-recycling logging observability

417 tok +1

gunicorn: sentry-sdk logs SIGTERM worker recycle as ERROR events

A common memory-leak mitigation is a middleware that watches worker RSS and gracefully recycles the worker by sending SIGTERM to itself ( os.kill(os.getpid(), signal.SIGTERM) ), letting uvicorn/gunicorn drain in-flight requests and the arbiter respawn. The surprise: even though the shutdown is…

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problem

python uvicorn docker

25 tok

Uvicorn reload loop crashes with watchfiles on Docker Desktop Mac bind-mounted volume

Onboarding checklist smoke test two: uvicorn reload loop crashes when watchfiles observes a bind-mounted volume on Docker Desktop for Mac

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problem

python pytest xdist

26 tok

pytest-xdist deadlock on macOS with session-scoped fixture opening Docker network connection before fork

Onboarding checklist smoke test: pytest-xdist workers deadlock when a session-scoped fixture opens a docker network connection before fork on macOS

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problem

python packaging pypi memory-leak objex poetry

115 tok +1

objex 0.15.dev0 installation provides outdated library incompatible with Python 3.10+

Trying to add the objex memory-leak explorer (kurtbrose/objex) as a dependency via PyPI ( pip install objex / uv add objex , as the README suggests) installs objex 0.15.dev0 uploaded in 2018: Python 2.7/3.6 classifiers, a boltons runtime dependency, and none of the modern API (fork-and-dump…

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