How to Use @pytest.fixture for Setup in Tests

Use @pytest.fixture to define reusable setup functions that provide test data or state. Mark a function with @pytest.fixture and pass it as an argument to your test functions to use the setup automatically.

Syntax

The @pytest.fixture decorator marks a function as a fixture. This function can prepare data or state for tests. Tests receive the fixture by naming it as a parameter.

• @pytest.fixture: Decorator to define a fixture.
• Fixture function: Returns data or sets up state.
• Test function: Accepts fixture name as argument to use it.

python

import pytest

@pytest.fixture
def sample_data():
    return [1, 2, 3]

def test_sum(sample_data):
    assert sum(sample_data) == 6

Output

============================= test session starts =============================
collected 1 item

test_sample.py .                                                          [100%]

============================== 1 passed in 0.01s ==============================

Example

This example shows a fixture that provides a list. The test uses this list to check the sum. The fixture runs before the test and passes its return value automatically.

python

import pytest

@pytest.fixture
def numbers():
    print("Setup: Creating numbers list")
    return [10, 20, 30]

def test_total(numbers):
    assert sum(numbers) == 60

def test_length(numbers):
    assert len(numbers) == 3

Output

============================= test session starts =============================
collected 2 items

Setup: Creating numbers list
.
Setup: Creating numbers list
.

test_example.py ..                                                         [100%]

============================== 2 passed in 0.01s ==============================

Common Pitfalls

Common mistakes include:

• Not using the fixture name as a test argument, so the fixture never runs.
• Modifying fixture data inside tests, which can cause unexpected results if reused.
• Using print statements expecting output without running pytest with -s option.

python

import pytest

@pytest.fixture
def data():
    return [1, 2, 3]

def test_wrong():
    # Fixture 'data' is defined but not used here
    assert True

def test_right(data):
    # Correct usage: fixture passed as argument
    assert sum(data) == 6

Output

============================= test session starts =============================
collected 2 items

test_pitfalls.py .                                                          [100%]

============================== 2 passed in 0.01s ==============================

Quick Reference

Remember these tips when using @pytest.fixture:

• Define setup code in a function decorated with @pytest.fixture.
• Use the fixture by naming it as a parameter in your test functions.
• Fixtures can return data or perform setup/teardown.
• Use scope parameter to control fixture lifetime (function, module, session).

Key Takeaways

Use @pytest.fixture to create reusable setup code for tests.

Pass fixture names as test function arguments to use them.

Fixtures can return data or perform setup tasks before tests run.

Avoid modifying fixture data inside tests to prevent side effects.

Use fixture scope to control how often setup runs.