Testing Asphalt components

Testing Asphalt components and component hierarchies is a relatively simple procedure:

  1. Create an instance of your Component

  2. Create a Context instance

  3. Run the component’s start() method with the context as the argument

  4. Run the tests

  5. Close the context to release any resources

With Asphalt projects, it is recommended to use the pytest testing framework because it is already being used with Asphalt core and it provides easy testing of asynchronous code (via the pytest-asyncio plugin).


Let’s build a test suite for the Echo Tutorial.

The client and server components could be tested separately, but to make things easier, we’ll test them against each other.

Create a tests directory at the root of the project directory and create a module named test_client_server there (the test_ prefix is important):

import pytest
from asphalt.core import Context

from echo.client import ClientComponent
from echo.server import ServerComponent

def test_client_and_server(event_loop, capsys):
    async def run():
        async with Context() as ctx:
            server = ServerComponent()
            await server.start(ctx)

            client = ClientComponent("Hello!")
            await client.start(ctx)

    with pytest.raises(SystemExit) as exc:

    assert exc.value.code == 0

    # Grab the captured output of sys.stdout and sys.stderr from the capsys fixture
    out, err = capsys.readouterr()
    assert out == "Message from client: Hello!\nServer responded: Hello!\n"

The test module above contains one test function which uses two fixtures:

  • event_loop: comes from pytest-asyncio; provides an asyncio event loop

  • capsys: captures standard output and error, letting us find out what message the components printed

In the test function (test_client_and_server()), the server and client components are instantiated and started. Since the client component’s start() function only kicks off a task that runs the client’s business logic (the run() method), we have to wait until the task is complete by running the event loop (using run_forever()) until run() finishes and its callback code attempts to terminate the application. For that purpose, we catch the resulting SystemExit exception and verify that the application indeed completed successfully, as indicated by the return code of 0.

Finally, we check that the server and the client printed the messages they were supposed to. When the server receives a line from the client, it prints a message to standard output using print(). Likewise, when the client gets a response from the server, it too prints out its own message. By using pytest’s built-in capsys fixture, we can capture the output and verify it against the expected lines.

To run the test suite, make sure you’re in the project directory and then do:

PYTHONPATH=. pytest tests

For more elaborate examples, please see the test suites of various Asphalt subprojects.