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The test client allows you to make requests against your ASGI application, using the requests library.

from starlette.responses import HTMLResponse
from starlette.testclient import TestClient


async def app(scope, receive, send):
    assert scope['type'] == 'http'
    response = HTMLResponse('<html><body>Hello, world!</body></html>')
    await response(scope, receive, send)


def test_app():
    client = TestClient(app)
    response = client.get('/')
    assert response.status_code == 200

The test client exposes the same interface as any other requests session. In particular, note that the calls to make a request are just standard function calls, not awaitables.

You can use any of requests standard API, such as authentication, session cookies handling, or file uploads.

By default the TestClient will raise any exceptions that occur in the application. Occasionally you might want to test the content of 500 error responses, rather than allowing client to raise the server exception. In this case you should use client = TestClient(app, raise_server_exceptions=False).

Selecting the Async backend

TestClient.async_backend is a dictionary which allows you to set the options for the backend used to run tests. These options are passed to anyio.start_blocking_portal(). See the anyio documentation for more information about backend options. By default, asyncio is used.

To run Trio, set async_backend["backend"] = "trio", for example:

def test_app()
    client = TestClient(app)
    client.async_backend["backend"] = "trio"
    ...

Testing WebSocket sessions

You can also test websocket sessions with the test client.

The requests library will be used to build the initial handshake, meaning you can use the same authentication options and other headers between both http and websocket testing.

from starlette.testclient import TestClient
from starlette.websockets import WebSocket


async def app(scope, receive, send):
    assert scope['type'] == 'websocket'
    websocket = WebSocket(scope, receive=receive, send=send)
    await websocket.accept()
    await websocket.send_text('Hello, world!')
    await websocket.close()


def test_app():
    client = TestClient(app)
    with client.websocket_connect('/') as websocket:
        data = websocket.receive_text()
        assert data == 'Hello, world!'

The operations on session are standard function calls, not awaitables.

It's important to use the session within a context-managed with block. This ensure that the background thread on which the ASGI application is properly terminated, and that any exceptions that occur within the application are always raised by the test client.

Establishing a test session

  • .websocket_connect(url, subprotocols=None, **options) - Takes the same set of arguments as requests.get().

May raise starlette.websockets.WebSocketDisconnect if the application does not accept the websocket connection.

websocket_connect() must be used as a context manager (in a with block).

Sending data

  • .send_text(data) - Send the given text to the application.
  • .send_bytes(data) - Send the given bytes to the application.
  • .send_json(data, mode="text") - Send the given data to the application. Use mode="binary" to send JSON over binary data frames.

Receiving data

  • .receive_text() - Wait for incoming text sent by the application and return it.
  • .receive_bytes() - Wait for incoming bytestring sent by the application and return it.
  • .receive_json(mode="text") - Wait for incoming json data sent by the application and return it. Use mode="binary" to send JSON over binary data frames.

May raise starlette.websockets.WebSocketDisconnect.

Closing the connection

  • .close(code=1000) - Perform a client-side close of the websocket connection.