Test is not executed when pytest.fixture is used

I have a simple Python library, for which I use the following command to run tests:

python setup.py pytest

The following test works as expected:

def test_out():
    assert 1 == 2

Test result:

platform linux -- Python 3.10.4, pytest-7.1.2, pluggy-1.0.0
rootdir: /media/drive2/src
collected 1 item                                                                                                                                                                           

tests/test_error.py F

However, when @pytest.fixture is added, the test is not executed:

import pytest

@pytest.fixture

def test_out():
    assert 1 == 2

Test result:

platform linux -- Python 3.10.4, pytest-7.1.2, pluggy-1.0.0
rootdir: /media/drive2/src
collected 0 items  

What is the reason for this behavior, and how can @pytest.fixture be added in a way that won’t prevent the test from running?

(I want to use capsys.readouterr(), so I believe @pytest.fixture is needed.)

>Solution :

First of all, pytest.fixture is a decorator, so it is affecting the function below, and the right way to use it is to put them together (no empty lines in between), like this:

import pytest

@pytest.fixture
def test_out():
    assert 1 == 2   # This still makes no sense, see answer below

@pytest.fixture just indicates that the return value of the decorated function will be use as parameter in other test function. (The decorated function will not run as a test).

From the pytest documentation:

“Fixtures”, in the literal sense, are each of the arrange steps and data. They’re everything that test needs to do its thing.

Here is an example similar to your test using a fixture:

import pytest

@pytest.fixture
def my_number():
    return 2

def test_out(my_number):
    assert 1 == my_number  # This will fail

When you create the fixture my_number the retorned value can be use as parameter in other test functions (in this case in test_out).

This is usefull when you have to create the same stuff in several tests. Instead of repeating the code in each test you can make a fixture.