📘 Overview

• What is it?: FastAPI is a modern, fast (high-performance), web framework for building APIs with Python 3.8+ based on standard Python type hints.
• Key Features:
  • Speed: Very high performance, on par with NodeJS and Go (thanks to Starlette and Pydantic).
  • Automatic Docs: Interactive API documentation (Swagger UI and ReDoc) generated automatically from code types.
  • Type-Safety: Built-in validation, serialization, and editor autocompletion powered by standard Python type hints.
  • Async Native: Supports async/await natively for non-blocking asynchronous requests.
• Installation:

  pip install fastapi uvicorn[standard]

🧾 Core Concepts

• Path Operations: Functions mapped to HTTP routes (e.g. @app.get("/items/{id}")).
• Data Validation (Pydantic): Request and response schemas are defined using Pydantic classes, ensuring strong data parsing and serialization.
• Dependency Injection: A robust system to inject dependencies (like database sessions, configurations, or security utilities) into route handlers.
• Async vs Sync: Route functions can be defined with async def (for non-blocking I/O) or standard def (which runs in an external threadpool).

💻 Common Code Patterns & Cheat Sheet

• Basic Application:

  from fastapi import FastAPI
   
  app = FastAPI()
   
  @app.get("/")
  async def read_root():
      return {"message": "Welcome to FastAPI"}

• Request Parameters & Pydantic Validation:

  from pydantic import BaseModel
  from typing import Optional
   
  class Item(BaseModel):
      name: str
      description: Optional[str] = None
      price: float
      tax: Optional[float] = None
   
  @app.post("/items/")
  async def create_item(item: Item):
      return {"item_name": item.name, "total_price": item.price + (item.tax or 0)}

• Dependency Injection Pattern:

  from fastapi import Depends, HTTPException, status
   
  def get_token_header(x_token: str = None):
      if x_token != "supersecrettoken":
          raise HTTPException(
              status_code=status.HTTP_400_BAD_REQUEST,
              detail="X-Token header invalid"
          )
      return x_token
   
  @app.get("/protected-route/")
  async def read_protected_data(token: str = Depends(get_token_header)):
      return {"secret_data": "FastAPI is awesome!", "token_used": token}

💡 Best Practices & Tips

• Run Command: Start the dev server using uvicorn main:app --reload (assuming filename is main.py).
• Async Judgement: Only use async def if you use an asynchronous library inside the route (like asyncpg or httpx). For blocking libraries (like standard sqlite3 or requests), use standard def to avoid blocking the main event loop.
• Response Model: Always specify response_model in path decorators (@app.get("/", response_model=Item)) to filter output data and enforce schema output structures.

🔗 Navigation & Internal Links

• Parent: Python
• Related Notes: Web Development | Api | Flask | SQLAlchemy