# Review of the code of MIL-004

## Metadata
| Key | Value |
| --- | --- |
| ID | RC-014 |
| CrossReference | [MIL-004], [QC-PY-001] |

## Version History
| Date | Status | Author | Reviewer | Change | Commit |
| --- | --- | --- | --- | --- | --- |
| 2026-10-28 | Proposed | Jens Tirsvad Nielsen | AI as stand-in for S01 | Initial version | [62cb520] |

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## Artifact Under Review

- Instance reviewed: [MIL-004] (the code of its pull request, `src/` and `tests/`)
- Checklist used: [QC-PY-001] (`QC-PY-001`)
- Scope: full review
- Language and domain: n/a
- Language reviewer: none
- Mode: autonomous, not independent (reviewer is `AI as stand-in for S01`)

## Checklist Results

| # | Criterion | Status | Evidence/Notes |
| --- | --- | --- | --- |
| 1 | Packages, modules, functions, variables, classes and constants follow PEP 8 casing (`snake_case`, `PascalCase`, `UPPER_SNAKE`) | Pass | PascalCase classes, snake_case functions, UPPER_SNAKE constants. |
| 2 | Names state purpose in the domain's language; no unexplained abbreviations, no single-letter names outside tiny scopes | Pass | Names state purpose (BrowserFileMissingError, count_library_playlists). |
| 3 | Code is produced by the project's formatter and passes its linter with no unexplained suppressions | Pass | ruff format and ruff check report no finding. |
| 4 | Every function and method signature is type-annotated, including `-> None` | Pass | All functions annotated; the one YTMusic default factory is typed by the YTMusicLike protocol. |
| 5 | No bare `except:`, no swallowed exceptions; specific exceptions are raised and the cause is kept (`raise ... from`) | Pass | Specific ytmusicapi errors are caught and re-raised as AuthenticationError with `from`. |
| 6 | No mutable default arguments and no shadowed builtins | Pass | Checked in the diff. |
| 7 | Files, locks and connections are managed with context managers | N-A | The code opens no file; it checks Path.exists and passes the path to ytmusicapi. |
| 8 | Public modules, classes and functions have docstrings that say what, not how | Pass | Checked in the diff. |
| 9 | Logging uses `logging`, not `print`; no secrets or personal data in log output | Pass | No print in the source except the injected writer; browser.json content is never read or logged. |
| 10 | Classes and operations trace to the Design Class Diagram they implement; deviations are recorded | N-A | No Design Class Diagram exists; all tasks are technical. |
| 11 | Tests exist for new behaviour, are named for the behaviour, and do not depend on order or the network | Pass | 47 tests, 97 % coverage, no network; the missing-file test uses tmp_path; browser.json is never created. |
| 12 | Type checker runs in strict mode without errors; `Any` is justified in a comment | N-A | mypy is not configured; no Any in the source. Optional criterion. |
| 13 | Dependencies are declared and pinned in the project's dependency file, none unused | Pass | ytmusicapi is now used by infrastructure. |
| 14 | Each piece of knowledge (a business rule, constant, format, validation or query) is defined in one place; code is merged only where it expresses the same knowledge, not where it merely looks alike | Pass | Messages and the file name are constants defined once. |
| 15 | Imports point inward: business-rule modules import no framework, database, UI or delivery-mechanism code, and there are no import cycles | Pass | The port is in application; only infrastructure imports ytmusicapi. |
| 16 | SOLID holds: a class or module has one reason to change; new behaviour is added by extension, not by editing a type test in several places; subclasses honour the contract of their base; protocols are narrow; high-level code depends on protocols or abstract types and receives its implementations | Pass | MusicService is a narrow Protocol; the CLI depends on it. |
| 17 | Source is laid out in the layers and folders of the clean architecture folder structure (`domain`, `application`, `infrastructure`, `presentation` packages under `src/`, and `unit`, `integration` and `e2e` under `tests/`), in `snake_case`, and imports between the layers point inward | Pass | Layer folders and tests/unit/<layer> exist. |
| 18 | File and folder names are English, contain no space and no character outside letters, digits, `-`, `_` and `.`, and follow the casing of the language for files | Pass | English snake_case file names. |

## Overall Verdict

Go — every Mandatory criterion passes or does not apply; no blocking finding.

## Action Items

| Action | Owner | Due |
| --- | --- | --- |
| None | S01 | - |

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[MIL-004]: ../../milestones/mil-004-step-2-youtube-music-auth.md
[QC-PY-001]: ../../../framework/qc/code/qc-programming-python.md
[62cb520]: https://git.tirsystem.com/Tirsvad-Udemy-100-days-of-code/046-scraping-the-billboard/commit/62cb520d1e856a97d3e63adee942274629751c38