Pytest Fixtures

What is a Pytest Fixture?

In Pytest, a fixture is a function that is used to prepare the test environment before a test is run. Fixtures are a powerful tool in Pytest because they allow you to reuse code, and they provide a way to set up and clean up test resources.

Fixture Syntax

To define a fixture in Pytest, you can use the @pytest.fixture decorator. Here is an example of a simple fixture that creates a list:

import pytest

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

You can also define fixtures with arguments. For example, here is a fixture that creates a list with a given number of elements:

import pytest

@pytest.fixture
def my_list(size):
    return [i for i in range(size)]

Using Fixtures in Tests

To use a fixture in a test, you can pass it as an argument to the test function. Here is an example of a test that uses the my_list fixture defined above:

def test_my_list(my_list):
    assert len(my_list) == 3

You can also use fixtures in test class methods by marking the fixture name as an argument in the @pytest.mark.usefixtures decorator. For example:

import pytest

@pytest.mark.usefixtures("my_list")
class TestClass:
    def test_method_1(self):
        assert len(self.my_list) == 3
    def test_method_2(self):
        assert self.my_list == [1, 2, 3]

Chaining Fixtures You can chain fixtures together by using one fixture as an argument to another fixture. For example, here is a fixture that creates a list of lists:

import pytest

@pytest.fixture
def nested_list(my_list):
    return [my_list, my_list]

In this case, the nested_list fixture depends on the my_list fixture. This means that the my_list fixture will be run before the nested_list fixture.

Fixture Scopes

Pytest fixtures have four possible scopes: function, class, module, and session.

The function scope is the default scope, and it means that the fixture is run before every test function that uses it.
The class scope means that the fixture is run once before the first test method in a class and once after the last test method in the class.
The module scope means that the fixture is run once before the first test in the module and once after the last test in the module.
The session scope means that the fixture is run once at the beginning of the test session and once at the end of the test session.

To specify a scope for a fixture, you can use the scope argument of the @pytest.fixture decorator. For example:

import pytest

@pytest.fixture(scope="module")
def my_module_fixture():
    # code to set up test resources
    yield
    # code to clean up test resources

@pytest.fixture(scope="session")
def my_session_fixture():
    # code to set up test resources
    yield
    # code to clean up test resources

conftest.py

You can use the conftest.py file to define fixtures that are available to all tests in a directory and its subdirectories. This is useful if you have a set of fixtures that you want to use in multiple test modules.

To define a fixture in the conftest.py file, you can use the same syntax as above. For example:

import pytest

@pytest.fixture
def my_common_fixture():
    # code to set up test resources
    yield
    # code to clean up test resources

Common Pitfalls to Avoid

Here are some common pitfalls to avoid when using Pytest fixtures:

Forgetting to use the yield statement: If you forget to use the yield statement in a fixture, the code after the yield statement will not be run. This means that any code you have to clean up test resources will not be run.
Using global variables to store fixture data: It is generally a good idea to avoid using global variables in your tests. Instead, use fixtures to pass data between tests.
Using fixtures with the wrong scope: Make sure to use the correct scope for your fixtures based on when you want them to run.

Best Practices

Here are some best practices for using Pytest fixtures:

Keep fixtures simple: Fixtures should be used to set up and clean up test resources, not to perform complex operations.
Use fixtures to reduce duplication: Use fixtures to avoid duplication of setup and cleanup code in your tests.
Use fixtures to make tests more readable: By using fixtures, you can make your tests more readable by reducing the amount of setup and cleanup code in your tests.