FastAPI TestClient not starting lifetime in test

Example code:

import os
import asyncio
from contextlib import asynccontextmanager
from fastapi import FastAPI, Request

@asynccontextmanager
async def lifespan(app: FastAPI):
    print(f'Lifetime ON {os.getpid()=}')
    app.state.global_rw = 0

    _ = asyncio.create_task(infinite_1(app.state), name='my_task')
    yield 

app = FastAPI(lifespan=lifespan)

@app.get("/state/") 
async def inc(request: Request):
    return {'rw': request.app.state.global_rw}

async def infinite_1(app_rw_state):
    print('infinite_1 ON')
    while True:
        app_rw_state.global_rw += 1
        print(f'infinite_1 {app_rw_state.global_rw=}')
        await asyncio.sleep(10) 

This is all working fine, every 10 seconds app.state.global_rw is increased by one.

Test code:

from fastapi.testclient import TestClient

def test_all():
    from a_10_code import app 
    client = TestClient(app)

    response = client.get("/state/")
    assert response.status_code == 200
    assert response.json() == {'rw': 0}

Problem that I have found is that TestClient(app) will not start async def lifespan(app: FastAPI):.
Started with pytest -s a_10_test.py

So, how to start lifespan in FastAPI TestClient ?

P.S. my real code is more complex, this is just simple example for demonstration purposes.

The main reason that the written test fails is that it doesn't handle the asynchronous nature of the FastAPI app's lifespan context properly. In fact, the global_rw is not set due to improper initialization.

If you don't want to utilize an AsyncClient like the one by httpx you can use pytest_asyncio and the async fixture, ensuring that the FastAPI app's lifespan context correctly works and global_rw is initialized properly.

Here's the workaround:

import pytest_asyncio
import pytest
import asyncio

from fastapi.testclient import TestClient
from fastapi_lifespan import app

@pytest_asyncio.fixture(scope="module")
def client():
    with TestClient(app) as client:
        yield client

@pytest.mark.asyncio
async def test_state(client):
    response = client.get("/state/")
    assert response.status_code == 200
    assert response.json() == {"rw": 1}

    await asyncio.sleep(11)

    response = client.get("/state/")
    assert response.status_code == 200
    assert response.json() == {'rw': 2}

You can also define a conftest.py to place the fixture there to have a clean test files.