Python Services & CLI

Working rules

• Validate external inputs and responses at boundaries; preserve exception causes.
• Keep missing results distinct from empty results. When an empty collection means success, require the response field instead of defaulting it to empty; an all-default model can validate {} as success. Test missing, misspelled, explicitly empty, and populated payloads through the real validation boundary.
• Keep simple sequential work synchronous. Bound concurrent work, preserve cancellation, and keep task references.
• Give network calls timeouts; retry only failures and operations whose semantics permit it.
• Enforce job idempotency with atomic writes and database constraints.
• Preserve published API behavior and shared migration history.
• Match telemetry to an operational question and verify its output.

Discipline

• Simplicity first: every change as simple as possible, impact minimal code
• Only touch what's necessary; avoid introducing unrelated changes
• No hacky workarounds: if a fix feels wrong, step back and implement the clean solution
• Before adding a new abstraction, verify it appears in 3+ places. If not, inline it.
• Verify: see Verify section below; pass all checks with zero warnings before declaring done

Verify

• uv run pytest passes with zero failures
• uv run ruff check . passes with zero warnings
• uv run ty check . passes with zero errors
• Coverage target: 80%+ (uv run pytest --cov; add --cov-report=html for a browsable report)

Task-specific references

Read the relevant reference before implementing or reviewing the matching behavior:

• For packaging, environment configuration, CLI setup, or pytest behavior: tooling-and-tests.md.
• For asyncio, background jobs, retries, pooling, timeouts, or health checks: concurrency-and-resilience.md.
• For APIs, validation, errors, migrations, logging, metrics, or traces: service-boundaries.md.

Existing specialized references, when the corresponding topic applies:

• cli-tools.md.
• fastapi.md.