| name | split-monolith |
| description | Safe procedure for decomposing a god file (400+ LOC) into a sub-package without breaking any imports. Load when a file exceeds 400 lines or mixes multiple concerns. Implements vibecodex Principles A1 and A8 — folder-instead-of-file with backward-compatible re-exports. |
split-monolith
A file split done wrong breaks every caller. Follow this procedure exactly — it is reversible at every step.
Step 0 — Confirm the file needs splitting
wc -l app/services/<file>.py
| Lines | Action |
|---|
| < 400 | Do not split — you're solving a non-problem |
| 400–600 | Plan the split now, execute when convenient |
| > 600 | Split immediately (Principle A7 hard cap) |
Also ask: does this file mix multiple concerns? A file that is long but cohesive is better than a premature split.
Step 1 — Identify the domain splits
Do NOT split by size. Split by type of responsibility.
Good splits (by domain):
wallet_service.py (1200 LOC) →
wallet/user.py ← user-facing operations (charge, refund)
wallet/admin.py ← admin operations (top-up, override)
wallet/history.py ← read-only queries
Good splits (by layer):
generation_service.py (1000 LOC) →
generation/orchestrator.py ← coordinates the flow
generation/cost.py ← cost calculation logic
generation/storage.py ← result persistence
Bad splits (by size only — don't do this):
big_service.py →
big_service_part1.py ← meaningless
big_service_part2.py ← meaningless
Write the target structure before touching any file.
Step 2 — Create the package directory
mkdir app/services/<domain>/
Do NOT move any code yet.
Step 3 — Create sub-files one at a time
For each sub-file, copy (not move) the relevant functions:
touch app/services/<domain>/user.py
Each sub-file must:
- Have its own imports (do not rely on
* imports)
- Be under 200 LOC (you're splitting — keep it lean)
- Contain one cohesive responsibility
Step 4 — Create __init__.py with ALL old names re-exported
This is the most important step. Every name that existed in the original file must still be importable from the same path.
from app.services.<domain>.user import CreditsUserService
from app.services.<domain>.admin import CreditsAdminService
from app.services.<domain>.user import get_credits_user_service
CreditsService = CreditsUserService
__all__ = [
"CreditsUserService",
"CreditsAdminService",
"CreditsService",
"get_credits_user_service",
]
Step 5 — Verify no import breaks
python3 -c "from app.services.<domain> import <OldClassName>"
python3 -c "from app.services.<domain> import <AnotherClass>"
pytest tests/ -x -q
All tests must be GREEN before deleting the original file.
Step 6 — Delete the original file
Only after Step 5 passes:
rm app/services/<original_file>.py
Run tests again:
pytest tests/ -x -q
bash scripts/lint-architecture.sh
Both must pass.
Common mistakes
| Mistake | Consequence | Prevention |
|---|
Split before writing __init__.py | Import errors everywhere | Always create __init__.py first |
| Split by size, not responsibility | Sub-files still coupled | Ask: "what is the single job of this file?" |
| Forget to re-export old names | Callers break silently | List every public name before splitting |
| Move code instead of copy+verify | Can't roll back | Copy first, delete only after tests pass |
| Split and refactor at same time | Impossible to debug | One PR = one split. No logic changes. |
Verification
The split was done correctly when: