polylith-migrate-extract-to-base

Development

[Internal sub-skill of `polylith-migrate-orchestrator`. Do not load directly — load `polylith-migrate-orchestrator` first, which drives all phases.] Extract all application code from `projects/<PROJECT>/` into a temporary migration base.

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Source SKILL.md: https://github.com/DavidVujic/python-polylith/blob/HEAD/.agents/skills/polylith/migrate-project/polylith-migrate-extract-to-base/SKILL.md

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Skill: polylith-migrate-extract-to-base

Goal

Extract all application code from projects/<PROJECT>/ into a temporary migration base (bases/<TARGET_TOP_NS>/<INITIAL_BASE_NAME>/).

Inputs

From migration/<PROJECT>/state.md:

PROJECT_DIR
ORIG_TOP_NS
TARGET_TOP_NS (default: ORIG_TOP_NS)
INITIAL_BASE_NAME
RUN_TEST_CMD (optional: RUN_LINT_CMD, RUN_TYPECHECK_CMD)

From migration/<PROJECT>/manifest.md:

Directory tree and module map.

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

Steps

1. Create the Base Directory

Create bases/<TARGET_TOP_NS>/<INITIAL_BASE_NAME>/.

2. Move Application Code

Move application packages/modules from projects/<PROJECT>/ to the base:
- For src/ layout: Move projects/<PROJECT>/src/<pkg>/ under the base.
- For flat layout: Move projects/<PROJECT>/<pkg>/ under the base.
Leave non-code files (Dockerfiles, k8s manifests, deploy scripts, pyproject.toml) in projects/<PROJECT>/.

3. Update `pyproject.toml`

Add the base to [tool.polylith.bricks]:

[tool.polylith.bricks]
"../../bases/<TARGET_TOP_NS>/<INITIAL_BASE_NAME>" = "<TARGET_TOP_NS>/<INITIAL_BASE_NAME>"

4. Fix Imports

Update imports minimally to ensure tests and linting pass.

5. Update `manifest.md`

Reflect the new structure in migration/<PROJECT>/manifest.md.

6. Handle Namespace Changes

If TARGET_TOP_NS != ORIG_TOP_NS, the migration orchestrator will handle this in subsequent phases:

polylith-migrate-generate-shim will create a compatibility shim at projects/${PROJECT}/${ORIG_TOP_NS}/__init__.py that re-exports from ${TARGET_TOP_NS}.${INITIAL_BASE_NAME}.
polylith-migrate-automate-import-updates will update imports in the new base location (at bases/${TARGET_TOP_NS}/${INITIAL_BASE_NAME}/) to reference the new namespace.
polylith-migrate-update-tests will update test files to use the compatibility shim.

7. Use Shims if Needed

If imports break during this phase, add minimal temporary shims to re-export names from the new brick API.
Track shims in migration/shims.md.
Note that comprehensive shim generation will be handled in the polylith-migrate-generate-shim phase.

Verify

RUN_TEST_CMD succeeds with the same pass/fail counts as the pre-migration baseline recorded in polylith-migrate-discover.
If set, RUN_LINT_CMD and RUN_TYPECHECK_CMD succeed.
Run POLY_CMD_PREFIX check to validate the workspace structure.
Run POLY_CMD_PREFIX sync to synchronize the [tool.polylith.bricks] table with actual imports.

Common failure modes

Symptom	Likely cause	Remediation
`ModuleNotFoundError: No module named '<ORIG_TOP_NS>.<x>'` after move	`TARGET_TOP_NS != ORIG_TOP_NS` and imports weren't rewritten or shimmed.	This is resolved by later phases per `SHIM_STRATEGY` (chosen in `polylith-migrate-analyze-imports`): `shimless` rewrites every reference in `polylith-migrate-automate-import-updates` (phase 4); `shim` adds a re-export shim under `ORIG_TOP_NS` in the phase 4b sub-track (`polylith-migrate-generate-shim`). To keep this phase green in the meantime, add a minimal temporary shim and record it in `migration/shims.md` (step 7).
`RUN_TEST_CMD` collects 0 tests after the move	Test files moved but `pytest` rootdir / `testpaths` still points at the old `projects/<PROJECT>/tests` location.	Update `pyproject.toml` `[tool.pytest.ini_options].testpaths` or pass explicit dirs in `RUN_TEST_CMD`. (Note: `polylith-migrate-prepare-project` moves tests properly — if extract-to-base touched tests at all, consider undoing that part.)
`poly check` reports "brick imports another brick that is not in `[tool.polylith.bricks]`"	Step 3 only added the base; imports inside the base may pull in components not yet listed.	Run `POLY_CMD_PREFIX sync --quiet` to populate the rest, then re-run `check`.
Editable install / build fails (`error: package directory '<x>' does not exist`)	The base move broke the previous `[tool.setuptools]` or `[tool.hatch.build]` `packages` setting in `projects/<PROJECT>/pyproject.toml`.	Update `packages` to point at the new `<TARGET_TOP_NS>` namespace, or remove the explicit `packages` setting and let Polylith's build hook handle it.
Verification fails and you can't quickly diagnose	Phase commit not yet made.	`git reset --hard HEAD` to roll back to the previous phase's commit and consult the user.

Commit

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

git add -A && git commit -m "migrate(<PROJECT>): phase <N> — extract-to-base"

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.