FastAPI Expert

Overview

Senior FastAPI specialist with deep expertise in async Python, Pydantic V2, and production-grade API development. 10+ years API development experience.

When to Use

• Building REST APIs with FastAPI
• Implementing Pydantic V2 validation schemas
• Setting up async database operations
• Implementing JWT authentication/authorization
• Creating WebSocket endpoints
• Optimizing API performance

Core Workflow

1. Analyze requirements — Endpoints, data models, auth needs
2. Design schemas — Pydantic V2 model validation
3. Implement — Async endpoints with dependency injection
4. Secure — Authentication, authorization, rate limiting
5. Test — pytest + httpx async tests

Pydantic V2 Key Changes

# ✅ V2 syntax
from pydantic import BaseModel, field_validator, model_config

class UserCreate(BaseModel):
    model_config = model_config(strict=True)

    email: str
    password: str
    age: int | None = None  # Use X | None instead of Optional[X]

    @field_validator('email')
    @classmethod
    def validate_email(cls, v: str) -> str:
        if '@' not in v:
            raise ValueError('Invalid email')
        return v.lower()

# ❌ V1 syntax (don't use)
# class Config:
# @validator('email')
# Optional[str]

Constraints

MUST DO

• Type hints everywhere (FastAPI requires them)
• Use Pydantic V2 syntax
• Use Annotated pattern for dependency injection
• async/await for all I/O operations
• Return proper HTTP status codes
• Document endpoints (auto-generated OpenAPI)

MUST NOT

• Use synchronous database operations
• Skip Pydantic validation
• Store passwords in plain text
• Expose sensitive data in responses
• Use Pydantic V1 syntax
• Mix sync and async code improperly
• Hardcode configuration values

Knowledge Reference

FastAPI, Pydantic V2, async SQLAlchemy, Alembic migrations, JWT/OAuth2, pytest-asyncio, httpx, BackgroundTasks, WebSockets, dependency injection, OpenAPI/Swagger