Here's a `main.py` file for a FastAPI backend named 'blog_app' with necessary imports and middleware, including SQLite and SQLAlchemy support:

import os
from typing import List
from fastapi import FastAPI, Depends, HTTPException, status
from fastapi.middleware.cors import CORSMiddleware
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, Session
from sqlalchemy.ext.declarative import declarative_base

SQLALCHEMY_DATABASE_URL = "sqlite:///./blog_app.db"
engine = create_engine(SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False})
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
Base = declarative_base()

app = FastAPI()

origins = [
    "http://localhost",
    "http://localhost:3000",

app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],

def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

from models import Base
Base.metadata.create_all(bind=engine)

from routers import blog, user

app.include_router(blog.router)
app.include_router(user.router)

@app.get("/")
def root():
    return {"message": "Welcome to the Blog App!"}

This `main.py` file sets up the necessary components for a FastAPI backend with SQLite and SQLAlchemy support. Here's a breakdown of what's included:

7. Creates database tables if they don't exist by calling `Base.metadata.create_all(bind=engine)`.
8. Imports routers for blog and user functionality (assuming you have separate router files).
10. Defines a root path (`/`) that returns a welcome message.

Note that this `main.py` file assumes you have separate files for models (`models.py`) and routers (`routers/blog.py` and `routers/user.py`). You'll need to create those files and define your models and routes accordingly.