FastAPI v0.128: Modern Python Web Framework for APIs

Quick Answer

FastAPI is Python’s premier framework for building high-performance APIs. By combining Starlette’s ASGI toolkit with Pydantic’s validation, it achieves performance comparable to Node.js/Go while maintaining Python’s developer-friendly syntax. Version 0.128.0 represents the stable branch, with automatic OpenAPI documentation, dependency injection, and WebSocket support out of the box.

What is FastAPI?

FastAPI is a modern, fast (high-performance) web framework for building APIs with Python 3.8+ based on standard Python type hints. It was created by Sebastián Ramírez (tiangolo) to address the need for a framework that could match the performance of Node.js and Go while leveraging Python’s ecosystem, particularly for data science and machine learning.

Version 0.128.0 Highlights

Key Features:

• Automatic OpenAPI (Swagger UI) documentation
• Pydantic v2 integration (validation, serialization)
• Async/await support (Starlette ASGI)
• Dependency Injection system
• WebSocket support
• Background task processing

Release Date: December 27, 2025

Maintainer: Sebastián Ramírez

Core Architecture

ASGI Foundation

Built on Starlette:

• ASGI (Async Server Gateway Interface)
• Uvicorn (reference server)
• Support for HTTP/2, WebSockets

Type Safety via Pydantic

from pydantic import BaseModel

class Item(BaseModel):
    name: str
    price: float
    is_offer: bool = False

@app.post("/items")
async def create_item(item: Item):
    return item

Benefits:

• Runtime validation (422 errors on invalid input)
• Automatic OpenAPI schema generation
• IDE autocomplete

Platform Support

Platform	Support	Details
Web	✅ True	JSON APIs, SSR via Jinja2
API	✅ True	Primary use case
Serverless	✅ True	AWS Lambda (Mangum)
Edge	✅ True	Cloudflare Workers (Python beta)

Key Features

1. Automatic Documentation

Swagger UI: /docs ReDoc: /redoc OpenAPI Schema: /openapi.json

2. Dependency Injection

from fastapi import Depends

async def get_db():
    return database_connection

@app.get("/users")
async def read_users(db = Depends(get_db)):
    return db.query("SELECT * FROM users")

3. Background Tasks

from fastapi import BackgroundTasks

def send_email(email: str):
    # Send email logic
    pass

@app.post("/signup")
async def_signup(email: str, background_tasks: BackgroundTasks):
    background_tasks.add_task(send_email, email=email)
    return {"message": "Signup successful"}

FastAPI vs Django

Feature	FastAPI	Django
Architecture	Micro-framework (DIY)	Batteries-included monolith
Async Support	Native (Starlette)	Limited (sync ORM)
API Docs	Automatic (OpenAPI)	Django REST Framework
Performance	High (async)	Moderate (sync)

Best Use Cases

Ideal For

• ML/AI inference APIs
• High-concurrency microservices
• Public APIs (auto-documentation)
• Data validation hubs

Avoid For

• Simple static websites (use Hugo/Jekyll)
• Monolithic CMS with admin (use Django)
• Teams with zero Python experience

Known Issues

Pydantic v2 Migration

• Breaking changes in validation logic
• Union type coercion issues

Async/Sync Pitfall

• Using async def with blocking code blocks event loop
• Solution: Use def for blocking operations

Conclusion

FastAPI v0.128 represents the state-of-the-art for Python API development. Its combination of type safety, automatic documentation, and async performance makes it the optimal choice for modern Python backend services, particularly those involving ML, data processing, or high-concurrency requirements.

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Last Updated: 2026-01-20