Use fixture instance in non-test classes

[peppe-russo]

Hello everyone, I'm writing a pytest plugin that uses pytest-vnc as a dependency. The way I use it is to wrap it around my own custom fixture to override the sleep method, like so:

# conftest.py
@pytest.fixture(scope='session')
def qt_vnc(vnc):
    vnc.sleep = sleep
    return vnc

I then have an Area class in another file that I use quite often around my plugin, which needs to use that VNC instance (or more precisely, the qt_vnc fixture), example:

# area.py
from dataclasses import dataclass, asdict
from pytest_vnc import RectLike

@dataclass
class Area(RectLike):
    x: int
    y: int
    width: int
    height: int

    def capture():
        qt_vnc.capture(**asdict(self))  # <- How can access qt_vnc here?

The current way I'm doing this is to have an optional vnc field to the Area class and just pass the fixture to it when I instanciate the class, which doesn't look very elegant and it's not always possible as I use this class from places where I can't easily access the fixture.

Is there a "proper" way to make the qt_vnc fixture like a global singleton or something like that so I can access it from my custom classes? (The Area class is not the only class that needs to use it).

Thanks in advance,
Peppe

[portyu9]

Hi @peppe-russo

A session-scoped fixture already gives every requester the same fixture instance, but pytest fixtures are not globally importable singletons.

The supported pattern is to keep the dependency explicit. If creating Area objects repeatedly is awkward, a factory fixture is a good way to hide that plumbing:

from dataclasses import dataclass

import pytest


@dataclass
class Area:
    x: int
    y: int
    width: int
    height: int
    vnc: object

    def capture(self):
        return self.vnc.capture(
            x=self.x,
            y=self.y,
            width=self.width,
            height=self.height,
        )


@pytest.fixture
def area_factory(qt_vnc):
    def create(**kwargs):
        return Area(vnc=qt_vnc, **kwargs)

    return create

Then:

def test_something(area_factory):
    area = area_factory(
        x=0,
        y=0,
        width=100,
        height=100,
    )
    area.capture()

Because qt_vnc is session-scoped, every area_factory request receives the same qt_vnc object.

request.getfixturevalue() exists for dynamically requesting fixtures from inside a pytest-managed test or fixture, but using it to make arbitrary classes pull dependencies out of pytest would effectively turn pytest into a service locator and tightly couple those classes to the test framework.

If many classes need VNC, another clean option is to create one fixture-backed service/context object containing the VNC dependency and have those objects receive that context.

Please mark this answer as accepted if it helped, thank you.