python-library-architecture

name	python-library-architecture
description	Design or review reusable Python library/package architecture with theme isolation, a side-effect-free `core`, and facade/client composition for multi-theme libraries and SDK-style packages.

Purpose

Choose clean internal architecture for reusable Python libraries and SDK-style packages so themes stay isolated, shared contracts live in core, and consumers enter through a clear facade/client.

Trigger / When to use

Use this skill when:

• designing or reviewing the internal architecture of a reusable Python library or SDK-style package
• deciding where themes, shared contracts, adapters, and the public facade/client should live
• refactoring a coupled package that has cross-theme imports, a bloated common/, or orchestration in the wrong layer

Do not use this skill when:

• the main task is package/distribution layout, src/, or pyproject.toml
• the main task is module export policy, __all__, or deep-import rules
• the main task is type-hint syntax, model-construct choice, serialization translation, or error policy
• the main task is service/application architecture, framework layering, or plugin design

Inputs

• the reusable library or SDK's consumer-facing capabilities
• the proposed themes or capability slices inside one package
• the contracts shared across themes or external boundaries
• the adapters or integrations that touch transport, storage, or vendor APIs
• the intended public facade/client entry points
• any current dependency smells, such as peer imports or a growing common/

Process

1. Confirm the task is library/package architecture only. If it is mainly about package layout, module gateways, typing syntax, model choice, serialization rules, error policy, or app/service architecture, hand off instead of widening scope.
2. Slice the library into themes that own one capability family each. Keep collaboration inside a theme by default.
3. Use core/ as the recommended shared contract center for multi-theme libraries. Allow a semantically explicit alternative such as kernel/ or base/ only if it plays the same role; do not use vague substitutes such as common/ or utils/.
4. Enforce a zero-exception cross-theme import ban. If two themes need the same contract, semantic primitive, or boundary-crossing abstraction, promote it into core instead of importing peer-to-peer.
5. Keep core side-effect-free and non-orchestrating. Put shared contracts, semantic value types, shared base errors, and pure support logic there; keep I/O, retries, auth refresh flows, client bootstrap, and other outward orchestration out of core.
6. Make the public facade/client the recommended composition root. Prefer one primary consumer-facing entry point; allow bounded secondary entry points only when they remain orchestration-only and do not become alternate architecture lanes.
7. Keep dependency direction explicit: facade/client and entry points may compose themes and adapters; adapters may depend on their owning theme and core; themes may depend on core; core must not depend outward.
8. Use the rules and checklist in reference.md as the portable baseline. Treat tools such as import-linter or Tach as optional verification aids, not as the architecture policy itself.
9. Use examples.md for pure-library, SDK-style, anti-pattern, and migration cases before recommending a refactor.

Examples

• Positive: Put shared pagination and credential contracts in core, keep storage/ and queues/ isolated from each other, and expose one Client that composes them without either theme importing the other.
• Negative: Let billing/ import users/models.py, move HTTP bootstrap into core/, or use common/ as a cross-theme dump zone.

Outputs

• a review-ready library-architecture rule set or design recommendation
• an explicit dependency-direction model for themes, core, adapters, and facade/client entry points
• refactor guidance for cross-theme coupling, core misuse, and composition-root cleanup

Verification

• confirm no theme imports another theme, with zero exceptions
• confirm core is the shared contract center and stays side-effect-free
• confirm shared contracts that cross themes or external boundaries are promoted into core
• confirm facade/client entry points compose themes and adapters instead of pushing orchestration down into core

Boundaries

• Do not define physical package/distribution layout; use python-package-layout.
• Do not define package gateways, __all__, deep-import policy, or internal-module contracts; use python-module-boundaries.
• Do not define type-hint syntax or strict-typing escape hatches; use python-type-hints-strict.
• Do not choose between Enum, dataclass, ABC, or Protocol; use python-model-selection.
• Do not define payload translation rules at API, database, or message boundaries; use python-serialization-boundaries.
• Do not define exception hierarchy or translation policy beyond allowing shared base errors in core; use python-error-handling.
• Do not widen into service/application architecture, framework-specific layering, or plugin systems.

Local references

• reference.md: hard rules, dependency-direction checklist, smell list, and optional-tooling notes for reusable library/package architecture
• examples.md: pure-library, SDK-style, anti-pattern, and migration examples for theme isolation, core, and facade/client composition