polylith-migrate-split-component-internals

Development

[Internal sub-skill of `polylith-migrate-orchestrator`. Do not load directly — load `polylith-migrate-orchestrator` first, which drives all phases.] Split monolithic `core.py` files in shared components into domain-focused modules.

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Source SKILL.md: https://github.com/DavidVujic/python-polylith/blob/HEAD/.agents/skills/polylith/migrate-project/polylith-migrate-split-component-internals/SKILL.md

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Skill: polylith-migrate-split-component-internals

📐 Scope vs sibling skills. This skill operates inside one already-extracted component, splitting its core.py into multiple sibling Python files. No new components are created — only the internal file layout changes. Don't confuse with:

polylith-migrate-split-big-component — splits one component into multiple components. Use that when the unit being broken up is a component, not a file.

polylith-migrate-isolate-shared-and-project-logic — separates shared vs project-specific code across components and projects; may create new components.

polylith-migrate-extract-standalone-modules — pulls foundational modules out of the residual big component into new standalone components.

Goal

Split monolithic core.py files in shared components (e.g., models_shared, schemas_shared, or similar) into domain-focused modules. This skill ensures that shared components remain well-organized and maintainable.

Inputs

From migration/<PROJECT>/state.md:

TARGET_TOP_NS
Verification commands.

From migration/<PROJECT>/manifest.md:

Current component list and structure.

All inputs from state.md are assumed to satisfy the validation rules in polylith-migrate-discover (### Validation rules). Validate before proceeding.

Steps

1. Identify Candidates

Scan components for large core.py files.
A component is a candidate if:
- The file contains definitions from multiple domains.
- The file contains helper/utility functions alongside class definitions.
- The file exceeds a reasonable size threshold.

2. Group Definitions by Domain

Group definitions by the domain concept they serve.
Example domains: ORM models, schema/dataclass clusters, helper functions.

3. Create New Modules

Create domain-focused modules (e.g., merchant.py, transaction.py).
Move relevant definitions from core.py to the new modules.

4. Update `init.py`

Re-export public names from the new modules in __init__.py.

5. Handle `core.py`

Delete core.py if all definitions have been moved.
Keep core.py if it serves as a composition point.

Verify

RUN_TEST_CMD succeeds.
If set, RUN_LINT_CMD and RUN_TYPECHECK_CMD succeed.

Common failure modes

Symptom	Likely cause	Remediation
`core.py` is small (<100 lines) but mixes two domains	The component itself is too small to warrant an internal split.	Leave it. Splitting adds indirection without benefit at this size; revisit when the file grows.
After splitting, an import like `from <ns>.<component> import <symbol>` fails	`__init__.py` was not updated to re-export the symbol from its new module.	Add `from <ns>.<component>.<new_module> import <symbol>` to `__init__.py`. The component's public API must remain stable across the split.
New files inside the component now circularly import each other (e.g., `models/user.py` ↔ `models/transaction.py`)	Domain split was too aggressive; the two files genuinely share a concept.	Extract the shared concept into a third file (e.g., `models/_base.py`) and have both depend on it.

Commit

After verification passes, commit this phase to the migration branch:

git add -A && git commit -m "migrate(<PROJECT>): phase <N> — split-component-internals"

Substitute <PROJECT>, <N>, and <phase-name> from state.md and the orchestrator's phase table. Do not proceed to the next phase without a clean commit — the per-phase commit is the rollback point for the next phase's failure-mode tables.