hello_world.py

# Import FastAPI to create the API
from fastapi import FastAPI

# Import uvicorn to run the app from this file
import uvicorn

# Create a FastAPI app instance
app = FastAPI(
    title="My First API",               # Title shown in docs
    description="A simple API using FastAPI",  # Short description
    version="1.0.0"                     # Version info
)

# Define the root route (homepage)
@app.get("/")  # This handles GET requests to "/"
def read_root():
    return {"message": "Hello World! Welcome to FastAPI"}  # JSON response

# Define a route that takes a name from the URL
@app.get("/hello/{name}")  # Handles GET requests like "/hello/Ali"
def read_item(name: str):  # Takes 'name' from the URL as a string
    return {"message": f"Hello, {name}!"}  # Personalized response

# This block runs the server if this file is executed directly
if __name__ == "__main__":
    # Start the server using uvicorn
    uvicorn.run("hello_world:app", host="127.0.0.1", port=8000, reload=True)
    # "main" is the filename (without .py), "app" is the FastAPI instance
    # reload=True allows auto-reloading on code changes