Testing Textual applications with pytest-bdd

[manoelpqueiroz]

Hi guys,

I am writing a Cookiecutter template to provide an initial setup for CLI/TUI apps. One of the features is the possibility of using pytest-bdd to write tests. The application is very simple, consisting only of a title screen:

from textual.app import App, ComposeResult
from textual.widgets import Footer, Label

class TerminalApp(App):
    """Textual app to serve as the project interface."""

    def compose(self) -> ComposeResult:
        """Create child widgets for the app."""
        yield Label("Project")
        yield Label("[i][b]Awesome description.[/]")
        yield Footer()

    def on_mount(self) -> None:
        """Execute instructions when launching the interface."""
        self.theme = "nord"

Reading the guide, testing this app is pretty straightforward and Pytest runs the test successfully:

from demo_app import TerminalApp

import pytest

@pytest.mark.asyncio
async def test_app_exit():
    interface = TerminalApp()

    async with interface.run_test() as pilot:
        await pilot.press("ctrl+q")

        assert interface.return_code == 0

Now, I am trying to port this test to a BDD framework, but having a lot of trouble since I'm not very familiar with async development. With a mix of my knowledge and some rounds with a LLM, I managed to come up with this code, which runs the entire test:

Feature: Main window

  Scenario: Quit the interface
    Given a terminal interface
    And the interface is running
    When the user presses `Ctrl+Q`
    Then the program is terminated without errors

from demo_app import TerminalApp

import pytest
from pytest_bdd import scenario, given, when, then, parsers

@scenario("main_window.feature", "Quit the interface")
def test_app_exit():
    pass

@pytest.fixture
def app_instance():
    return TerminalApp()

@pytest.fixture
@pytest.mark.asyncio
async def pilot(app_instance):
    async with app_instance.run_test() as pilot:
        yield pilot

@given("a terminal interface")
def terminal_interface(app_instance):
    # Here, the application is already instantiated from our fixture
    pass

@given("the interface is running")
def running_interface(pilot):
    # Here, pilot is already the running instance from our fixture
    pass

@when("the user presses `Ctrl+Q`")
@pytest.mark.asyncio
async def press_key(pilot):
    await pilot.press('ctrl+q')

@then("the program is terminated without errors")
def successful_termination(app_instance):
    assert app_instance.return_code == 0

It seems all steps manage to run, but the test fails as app_instance.return_code returns None by the end, indicating the Textual app has not been exited:

――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――― test_app_exit ―――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――
../../../.cache/pypoetry/virtualenvs/demo-rt9-1QUl-py3.13/lib/python3.13/site-packages/_pytest/fixtures.py:929: in call_fixture_func
    fixture_result = fixturefunc(**kwargs)
                     ^^^^^^^^^^^^^^^^^^^^^
tests/tui/test_main_window.py:44: in successful_termination
    assert app_instance.return_code == 0
E   AssertionError: assert None == 0
E    +  where None = TerminalApp(title='TerminalApp', classes={'-dark-mode'}, pseudo_classes={'focus', 'dark'}).return_code

I'm pretty sure this is just a matter of a minor fix, but being inexperienced with async functions, I must confess I am lost here. Would anyone be willing to help fixing this test? If you also have recommendations of good resources so I can finally nail how to deal with async tasks in Python, that would be very much appreciated as well! 🙂

[edward-jazzhands]

This is an interesting experiment and I commend you for it. I can't really speak to this particular issue, but I would recommend just becoming more familiar with how to write regular Textual tests first before diving into writing BDD tests. If as you said you are not experienced with async functions at all, I think it would be better to start there and work upwards.

The FastAPI docs have an article about async programming that is very well known among the Python Community, I highly recommend it if you haven't read it:

https://fastapi.tiangolo.com/async/

[manoelpqueiroz]

Thanks for the resource!

After digging some more, I have found that pytest-bdd and pytest-asyncio are unable to work together natively, and haven't been for some time (pytest-dev/pytest-bdd#223 and pytest-dev/pytest-asyncio#195).

I will be closing the discussion following these findings, thanks again for your help!