How do you run a basic Uvicorn server from the command line?

Uvicorn is started from the command line by specifying the application in module:attribute format. The module is the Python file (without .py), and the attribute is the ASGI application object inside it.

# File: main.py
async def app(scope, receive, send):
    assert scope['type'] == 'http'
    await send({
        'type': 'http.response.start',
        'status': 200,
        'headers': [(b'content-type', b'text/plain')],
    })
    await send({
        'type': 'http.response.body',
        'body': b'Hello, world!',
    })

# Run from the terminal:
# uvicorn main:app

# Basic run - listens on 127.0.0.1:8000
uvicorn main:app

# Custom host and port
uvicorn main:app --host 0.0.0.0 --port 8080

# Development mode with auto-reload
uvicorn main:app --reload

# FastAPI example
# File: api.py
from fastapi import FastAPI
app = FastAPI()

@app.get("/")
def read_root():
    return {"Hello": "World"}

# Run FastAPI app
uvicorn api:app --reload

# Nested attribute path
# File: mypackage/server.py with app = FastAPI()
uvicorn mypackage.server:app

The application path follows Python import syntax: dots separate packages, the colon separates the module from the attribute. If your ASGI application lives at mypackage/server.py and is named app, you write mypackage.server:app.

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