Modern Python Development

Overview

This skill provides modern Python development guidance for 2025, specializing in:

• LLM Applications: Structured outputs with Pydantic/Instructor
• API Development: FastAPI with type safety and dependency injection
• Clean Architecture: Thin, pragmatic application of hexagonal patterns
• Modern Tooling: uv, Ruff, mypy, pytest-watch

Designed for TypeScript developers who value simplicity and type safety.

Core Philosophy

Apply these principles to all Python development:

1. KISS (Keep It Simple): Simple is better than complex
2. YAGNI (You Aren't Gonna Need It): Implement only what's needed now
3. Boy Scout Rule: Leave code cleaner than you found it
4. Reader-Friendly Code: The next engineer should understand immediately

When to Use This Skill

Claude should automatically use this skill when:

• Setting up new Python projects
• Designing architecture for LLM applications
• Making decisions about project structure
• Choosing between architectural patterns
• Configuring development tooling (uv, Ruff, mypy)
• Implementing FastAPI endpoints
• Structuring tests (unit/integration/functional)
• Advising TypeScript developers on Python equivalents

Architecture Decision Framework

Step 1: Assess Complexity

Choose structure based on project complexity:

Step 2: Apply Thin Clean Architecture

Only add layers when complexity warrants it.

domain/     → Pure business logic (Pydantic models, services)
ports/      → Interfaces (Protocol classes) - add when needed
adapters/   → External integrations (API, DB, LLM clients)

Key principle: Start simple, refactor when pain points emerge.

Step 3: Choose Project Structure

• Single Package: Use for most projects (PoC, small-medium apps)
• Monorepo (uv workspaces): Use when sharing code across multiple services

See [REFERENCE.md](REFERENCE.md#project-structure) for detailed structures.

Development Workflow

Initial Setup

# 1. Initialize project
uv init my-project && cd my-project

# 2. Add dependencies
uv add fastapi pydantic uvicorn
uv add --dev pytest pytest-watch ruff mypy

# 3. Configure tools
# See REFERENCE.md for pyproject.toml configs

TDD Workflow

# Terminal 1: Watch tests (auto-run on changes)
uv run ptw -- tests/unit -v

# Terminal 2: Write code
# Follow Red -> Green -> Refactor cycle

Quality Checks

uv run ruff check --fix .  # Lint & auto-fix
uv run mypy src/           # Type check
uv run pytest --cov=src    # Test with coverage

LLM Application Patterns

Type-Safe Structured Outputs

Always use Pydantic for LLM outputs to ensure type safety:

from pydantic import BaseModel, Field

class ExtractedInfo(BaseModel):
    summary: str = Field(description="Brief summary")
    key_points: list[str]
    sentiment: Literal["positive", "neutral", "negative"]

Use Instructor for type-safe LLM calls:

import instructor
from openai import OpenAI

client = instructor.from_openai(OpenAI())
response = client.chat.completions.create(
    model="gpt-4o",
    response_model=ExtractedInfo,
    messages=[{"role": "user", "content": text}]
)
# response is typed as ExtractedInfo ✓

See [EXAMPLES.md](EXAMPLES.md#llm-patterns) for more patterns.

FastAPI Best Practices

Dependency Injection Pattern

from fastapi import FastAPI, Depends

def get_service() -> UserService:
    # Initialize dependencies here
    return UserService(repo=get_repo())

@app.post("/users")
async def create_user(
    request: CreateUserRequest,
    service: UserService = Depends(get_service)
):
    return service.create(request)

See [EXAMPLES.md](EXAMPLES.md#fastapi-patterns) for complete examples.

Testing Strategy

3-Layer Test Structure

tests/
├── unit/          # Fast, isolated, no I/O
├── integration/   # With real dependencies (DB, cache)
└── functional/    # E2E API tests

Coverage targets:

• Unit: 80%+ (business logic)
• Integration: Key flows only
• Functional: Critical user journeys

See [EXAMPLES.md](EXAMPLES.md#testing-examples) for test code.

Do's and Don'ts

✅ Do

• Use type hints everywhere (-> None, list[str], dict[str, Any])
• Prefer Protocol over ABC for interfaces
• Use pydantic.BaseModel for data classes with validation
• Run ruff check --fix before commits
• Keep functions small and focused

❌ Don't

• Over-engineer with unnecessary abstraction layers
• Use Any type unless absolutely necessary
• Skip type hints "for now"
• Create classes when functions suffice
• Add dependencies without clear need

TypeScript Developer Quick Reference

See [EXAMPLES.md](EXAMPLES.md#typescript-python-mapping) for detailed mapping.

Output Guidelines

When providing guidance:

1. Explain the "why" - Rationale behind recommendations
2. Show concrete examples - Runnable code snippets
3. Suggest next steps - What to implement/consider next
4. Flag complexity decisions - When to add vs skip abstraction

Always prioritize working code over perfect architecture.

References

• [REFERENCE.md](REFERENCE.md) - Tool configurations, project structures, commands
• [EXAMPLES.md](EXAMPLES.md) - Code examples, patterns, TypeScript mapping

Checklist for New Projects

• Project initialized with uv init
• Dependencies added via uv add
• pyproject.toml configured (Ruff, mypy, pytest)
• Project structure matches complexity level
• Type hints added to all functions
• Tests written (at least unit tests)
• TDD workflow established (pytest-watch)
• Pre-commit hooks considered (optional)