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Cosmic Python: Code Architecture for Production Systems

⚠️ CRITICAL DISTINCTION

Cosmic Python is NOT about system architecture (service boundaries, deployment topology, C4 models—that's the separate architecture skill).

Cosmic Python IS about code structure within a Python module or service: how to organize classes, functions, tests, and dependencies so that code is clean, testable, and maintainable.

When to use Cosmic Python:

• You have a service/module to build (the strategic scope is clear from architecture decisions)
• You need to structure the Python code inside that service with clean layers
• You want to enforce clean code principles, SOLID, and dependency management
• You need comprehensive test coverage organized per layer

This skill pairs with:

• Stream Coding (documentation-first methodology) – Use to plan WHAT to build
• Cosmic Python (this skill) – Use to structure HOW to build the code
• Architecture skill – Use for system-level design (separate scope)

THE MEANINGFY CONTRACT: MINIMISE WTFs PER MINUTE

Our goal is clean code that passes code review quickly while keeping developers productive and safe.

This is achieved through three non-negotiable commitments:

1. Layered Architecture – Strict Separation of Concerns

Within each Python module or service, separate code into four tightly-bounded layers:

• models/ – Domain logic (business rules, entities, value objects)

  • No I/O, no framework dependencies, pure domain
  • Tests focus on business invariants and domain rules
  • Fastest tests, run first

• adapters/ – Infrastructure and integration (databases, APIs, file systems)

  • Implement repositories, gateways, clients
  • Depend on models only; never on services or entrypoints
  • Tests mock external services; focus on integration boundaries

• services/ – Use-case orchestration (application logic, workflows)

  • Choreograph models and adapters
  • Contain transaction boundaries and error handling policy
  • May depend on models and adapters, never on entrypoints
  • Tests use mocks for adapters; focus on orchestration logic

• entrypoints/ – Request/response boundaries (API, CLI, schedulers, workers)

  • Parse input, call services, format responses
  • Handle routing, status codes, argument validation
  • Minimal business logic; delegate to services
  • Tests verify contracts (status codes, response shapes, argument parsing)

2. Dependency Direction – Always Enforced (DIP)

The Law:

entrypoints → services → models
              ↘
              adapters → models

Never the reverse. High-level policy (models + services) must never depend on low-level details (adapters). Low-level details depend on abstractions (interfaces/protocols), not the other way around.

When you violate this:

• Circular dependencies appear
• Models become framework-dependent
• Testing becomes hard (can't mock cleanly)
• You've broken the architecture

How to enforce:

• Use importlinter in CI/CD to block forbidden imports (see references)
• Code review checklist: "What imports what?" before approving

3. No Clean Code Without Tests – Every Layer Owns Its Tests

"If you can't test it, you can't understand it. If you can't understand it, you can't maintain it."

• 80%+ coverage on production code (target per layer)
• Tests per layer: Each layer tests its own responsibility, not other layers' internals
• TDD/BDD: Write tests before code; BDD (Gherkin) for use cases
• Test pyramid: Many unit tests (fast, isolated), fewer integration tests, minimal end-to-end

CORE PRINCIPLES FROM MEANINGFY ENGINEERING

Clean Code Standards

• Intention-revealing names – calculate_customer_tier() not calc_tier()
• Small, cohesive functions – <25 lines is a good target; if you say "and" in description, split
• No magic strings – Define constants or enums; raw string literals are technical debt
• Minimal nesting – Deep nesting signals complexity; refactor
• DRY but not obsessively – Three similar lines don't warrant a helper; ten do

SOLID Principles Applied Systematically

• SRP – One responsibility per class/function; if multiple reasons to change, split
• OCP – Extend via new classes/strategies, not conditional logic in existing code
• LSP – Subclasses must respect contracts; avoid overrides that break expected behavior
• ISP – Avoid "fat" interfaces; split into cohesive ones matching clients' needs
• DIP – Depend on abstractions (interfaces/protocols), inject concrete implementations

Observability as First-Class

• Structured logging – Logs are data, not strings; emit JSON with context
• Keep it in the right layers – Observability belongs in services and entrypoints, not deep in models
• OpenTelemetry patterns – Tracing and metrics via standard conventions
• No print statements in production – Use logging framework always

ONE-MINUTE CODE STRUCTURE CHECK

For each function/class, ask:

• Which layer does this belong in?
• What should it depend on?
• What should depend on it?
• What tests does it need?
• Does it have only one reason to change?

If you can't answer these clearly, the architecture is drifting. See the references for detailed refactoring paths.

COMMON WORKFLOWS

Workflow 1: Implementing Code From Specs (Stream Coding Phase 3)

When you have clear specs from Stream Coding Phase 2 (doc is 9+/10 Clarity Gate):

1. Identify which layer each component belongs to – Models? Services? Adapters?
2. Start with models/ – Pure domain logic, no I/O, no frameworks
3. Write unit tests for models first – Verify domain rules before implementing services
4. Implement adapters/ – Repositories, gateways, clients; mock external services in tests
5. Implement services/ – Orchestrate models and adapters; test with mocked adapters
6. Implement entrypoints/ – CLI, API, schedulers; minimal logic, mostly delegation
7. Run full test suite – Verify 80%+ coverage per layer
8. Run architectural checks – make check-architecture to validate import contracts

Workflow 2: Code Review Against Cosmic Python Standards

Before approving a pull request, ask:

Layering:

• Does models/ import from services, adapters, or entrypoints? ❌ Should not
• Do adapters/ import from services or entrypoints? ❌ Should not
• Do services/ import from entrypoints? ❌ Should not
• Is business logic in services/ or models/, not scattered in entrypoints/? ✅ Should be

Clean Code:

• Are functions <25 lines? ✅ Target for readability
• Do all functions have only one reason to change? ✅ Check for SRP violation
• Are all symbolic identifiers (role names, status values, etc.) constants/enums, not magic strings? ✅ Required
• Is there any deep nesting (3+ levels)? ❌ Refactor to smaller functions

Testing:

• Does each layer have appropriate unit tests? ✅ Required
• Are important decisions (branches, error cases) tested? ✅ All branches covered
• Do adapters use mocks for external systems? ✅ Tests must be isolated
• Is coverage reported per layer? ✅ Verify with coverage report

Observability:

• Are logs in services/ and entrypoints/, not deep in models/? ✅ Correct placement
• Is logging structured (JSON context), not formatted strings? ✅ Required for production

Workflow 3: Test Organization Per Layer

For each layer, write tests that match its responsibility:

Target: 80%+ coverage overall, with focus on each layer's responsibility (not mixing concerns).

Workflow 4: Spotting and Fixing Architecture Drift

Common anti-patterns and how to fix them:

QUALITY ASSURANCE & TOOLING

Essential Tools for Well-Guarded Projects

• Poetry – Dependency management, lockfiles, reproducible builds
• pytest + pytest-bdd – Unit tests (TDD) and behavior specs (BDD/Gherkin)
• tox – Test multiple Python versions in isolated environments
• importlinter – Enforce architectural boundaries (block forbidden imports)
• pylint / flake8 – Static analysis, style compliance
• SonarQube / SonarCloud – Code quality gates, duplication, security smells
• Codecov – Coverage reporting, trend tracking, gate on new code

Typical Makefile Pattern

make install                # Set up environment (poetry install)
make test                   # Run unit tests with coverage
make test-bdd               # Run BDD feature tests
make check-architecture     # Validate import contracts (importlinter)
make lint                   # Run style checks (pylint, flake8)
make ci                     # Full pipeline (all above)

CI/CD Workflow

Before merging to main:

1. Run tests and verify 80%+ coverage on new code
2. Run importlinter to block dependency violations
3. Run SonarCloud analysis; no new critical issues
4. Verify Clarity Gate from spec still matches code

PRINCIPLES & BEST PRACTICES

On Code Structure

• Organize by domain, not by layer – billing/models/, billing/services/, not models/billing/
• Use constants/enums for all symbolic identifiers – Never rely on "magic strings"
• Small, cohesive functions – If you say "and" when describing what a function does, split it
• Avoid deep nesting – >3 levels usually signals refactoring opportunity
• DRY but pragmatically – 2-3 similar lines are OK; 5+ warrant extraction

On Testing

• Test the decisions, not the steps – Test what varies (branches), not happy paths
• Test per layer, not per unit – Models test domain rules; services test orchestration
• Use fixtures and parametrization – pytest.mark.parametrize for multiple scenarios
• Mock external dependencies – Adapters provide mocks for services tests
• BDD for use cases – Gherkin feature files for end-to-end workflows

On Architecture Decisions

• Document major choices in ADRs – Architecture Decision Records explain the trade-off
• 5–8 ADRs per service is typical – More suggests decisions are tangled
• Contract-first design – Write OpenAPI/AsyncAPI specs before code
• Use dependency injection – Never import concrete implementations directly in services

On Observability

• Structured logging – Emit JSON with context, not formatted strings
• Keep observability in the right layers – services/ and entrypoints/, not deep in models/
• No print() in production – Use logging framework; configure per environment
• OpenTelemetry for tracing – Standard conventions for metrics and spans

WHEN NOT TO USE COSMIC PYTHON

• Rapid prototypes – Layering has upfront cost; worthwhile only if long-term maintenance matters
• Exploratory/spike code – Keep spikes separate; migrate to Cosmic Python only when strategy is clear
• Simple scripts – Single-file scripts don't need four layers

This approach requires team discipline. One developer ignoring layers breaks the architecture for everyone. All developers must respect boundaries and code review rigorously.

COSMIC PYTHON + STREAM CODING: THE COMPLETE WORKFLOW

Stream Coding (documentation-first planning) and Cosmic Python (clean code structure) work together:

The Integration

• Phase 2 specs must reference Cosmic Python layers – "Models layer will contain...", "Services layer will orchestrate..."
• Phase 3 code follows Cosmic Python patterns – Layering, SOLID, testing per layer
• Phase 4 maintenance – When fixing bugs, update spec first, then regenerate code (not manual patches)

REFERENCE MATERIALS

Layer-by-Layer Real-World Examples

• example-models-layer.md – Domain entities, value objects, Pydantic models, no I/O
• example-adapters-layer.md – Repositories, gateways, abstract interfaces, test doubles
• example-services-layer.md – Use-case orchestration, dependency injection, error handling patterns
• example-entrypoints-layer.md – FastAPI endpoints, Typer CLI, scheduler integration

Testing & Quality

• example-testing-patterns.md – Unit tests per layer, mocking strategy, fixtures, BDD patterns
• example-project-quality-ci.md – Makefile, tox, importlinter, GitHub Actions, SonarCloud setup

Advanced Production Patterns

• example-advanced-patterns.md – Configuration via decorators, exception hierarchies, protocols, cross-version services

Documentation-First Methodology (Stream Coding)

• stream-coding-methodology.md – 40/40/20 time split, 4 mandatory spec sections, Clarity Gate, Rule of Divergence
• phase-1-strategic-blueprint-checklist.md – 7 Questions framework, Strategic Blueprint, ADR writing
• phase-2-clarity-gate-checklist.md – AI-ready specs, 13-item Clarity Gate, scoring rubric

Company Standards & Principles

• MEANINGFY_PROMPT.md – Full Meaningfy engineering culture: project structure, layering rules, SOLID principles, CI/CD expectations, security

SKILL RELATIONSHIPS

• architecture ← System-level design (C4 models, service boundaries, deployment); use BEFORE Cosmic Python
• stream-coding ← Documentation-first methodology (planning specs); use WITH Cosmic Python for Phase 3 implementation
• cosmic-python ← Code structure within services (this skill)
• Your CI/CD tooling → importlinter, SonarCloud, pytest, tox (configured per Cosmic Python standards)