How do you test a Uvicorn-served ASGI application without starting the server?

For unit and integration testing, you don't want to actually start a server and make HTTP requests. The httpx library's ASGITransport (or Starlette's TestClient) allows you to test ASGI applications in-process - no socket, no port, no server startup required.

# Using httpx.AsyncClient with ASGITransport (pure async test)
import pytest
import httpx
from main import app

@pytest.mark.anyio
async def test_homepage():
    async with httpx.AsyncClient(
        transport=httpx.ASGITransport(app=app),
        base_url="http://test"
    ) as client:
        response = await client.get("/")
    assert response.status_code == 200
    assert response.json() == {"message": "Hello World"}

# Using Starlette / FastAPI TestClient (sync interface, uses anyio)
from fastapi.testclient import TestClient
from main import app

client = TestClient(app)

def test_root():
    response = client.get("/")
    assert response.status_code == 200
    assert response.json() == {"message": "Hello World"}

# Testing lifespan events with TestClient
def test_with_lifespan():
    with TestClient(app) as client:
        # lifespan startup has run at this point
        response = client.get("/")
        assert response.status_code == 200
    # lifespan shutdown has run after the with block

Why this works: ASGI applications are just async callables - they can be called directly in tests without a server. The transport layer (httpx or Starlette's TestClient) converts Python test calls into ASGI scope/receive/send messages, running the application synchronously in the test process.

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