/solid-python

name: solid-python
description: "Use when writing or refactoring Python code, structuring modules/[feature]/ layers, or defining Protocol-based interfaces (SOLID, files < 100 lines)."
versions:
  python: "3.12"
user-invocable: true
references: references/principles.md, references/patterns.md
related-skills: solid-detection

<objective> SOLID Python enforces a modular architecture for Python 3.12+: every feature lives under `modules/[feature]/` (routes, services, repositories, interfaces, models) with shared code in `core/`, interfaces are defined with `typing.Protocol` and live only in `interfaces/` directories, type hints are mandatory on every signature, and every public function carries a docstring.

Before writing any new code it requires a DRY check against `core/services` and `core/interfaces`. See `principles.md` for the 5 SOLID principles and `patterns.md` for directory layout, testing, and typing conventions. </objective>

SOLID Python - Modular Architecture

Agent Workflow (MANDATORY)

Before ANY implementation, spawn 3 agents in parallel, one `Agent` call each with a `name`:

1. **fuse-ai-pilot:explore-codebase** - Analyze existing architecture 2. **fuse-ai-pilot:research-expert** - Verify Python docs via Context7 3. **fuse-ai-pilot:sniper** - Post-implementation validation

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DRY - Reuse Before Creating (MANDATORY)

**Before writing ANY new code:** 1. **Grep the codebase** for similar interfaces, services, or logic 2. Check shared locations: `core/services/`, `core/interfaces/` 3. If similar code exists -> extend/reuse instead of duplicate 4. If code will be used by 2+ features -> create it in `core/`

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Architecture (Modules MANDATORY)

| Layer | Location | Max Lines | |-------|----------|-----------| | Routes | `modules/[feature]/routes/` | 50 | | Services | `modules/[feature]/services/` | 100 | | Repositories | `modules/[feature]/repositories/` | 100 | | Interfaces | `modules/[feature]/interfaces/` | 30 | | Models | `modules/[feature]/models/` | 50 | | Shared | `core/{services,interfaces,models}/` | - |

**NEVER use flat `src/` structure - always `modules/[feature]/`**

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Critical Rules (MANDATORY)

| Rule | Value | |------|-------| | File limit | 100 lines (split at 90) | | Routes | < 50 lines, delegate to services | | Interfaces | `interfaces/` directory ONLY | | Docstrings | Every public function documented | | Type hints | MANDATORY on all signatures | | Protocols | Use `typing.Protocol` for interfaces | | Small interfaces | 1-3 methods max |

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Reference Guide

| Topic | Reference | When to consult | |-------|-----------|-----------------| | **SOLID Principles** | [principles.md](references/principles.md) | Quick reference for all 5 principles | | **Patterns & Structure** | [patterns.md](references/patterns.md) | Directory layout, testing, typing |

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Forbidden

| Anti-Pattern | Fix | |--------------|-----| | Files > 100 lines | Split at 90 | | Interfaces in impl files | Move to `interfaces/` directory | | Fat interfaces (4+ methods) | Split into small Protocols | | Flat `src/` structure | Use `modules/[feature]/` | | Concrete dependencies | Use Protocol + dependency injection | | Missing type hints | Add type annotations to all signatures |