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 py.test 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 asyncio

import pytest
from asphalt.core import Context

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

def event_loop():
    # Required on pytest-asyncio v0.4.0 and newer since the event_loop fixture provided by the
    # plugin no longer sets the global event loop
    loop = asyncio.new_event_loop()
    yield loop

def context(event_loop):
    with Context() as ctx:
        yield ctx

def server_component(event_loop, context):
    component = ServerComponent()

def test_client(event_loop, server_component, context, capsys):
    client = ClientComponent('Hello!')
    exc = pytest.raises(SystemExit, event_loop.run_forever)
    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 (test_client) and three fixtures:

  • event_loop: provides an asyncio event loop and closes it after the test
  • context provides the root context and runs teardown callbacks after the test
  • server_component: creates and starts the server component

The client component is not provided as a fixture because, as always with CLIApplicationComponent, starting it would run the logic we want to test, so we defer that to the actual test code.

In the test function (test_client), the client component is instantiated and started. Since the component’s start() function only kicks off the 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:

pytest tests

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