How to Avoid Repeated Instantiation of a Class in Each Pytest Unit Test

The aim of this page📝 is to share how using @pytest.fixture decorator can help avoid the repetition of class instantiation in Pytest tests. I find it syntactically a bit cumbersome as you access your class through a decorated function, but it's still useful.

Pavol Kutaj

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Sep 14, 2023

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import pytest
import example_module

#ExampleClass defined in example_module

@pytest.fixture
def client():
    return example_module.ExampleClass(TEST_CLIENT_TAG, TEST_TICKET_ID)

def test_class_contains_expected_data(client):
    assert client.client_tag == "com_foobar"
    assert client.org_id == "bbc19d7f-b45b-4dad-b9bd-34a818f23241"
    assert client.ticket_id == "EXAMPLE-1234"
    assert client.dev_server == "com-foobar-dev1.net"
    assert client.prod_server == "com-foobar-prod1.net"

def test_read_keys_returns_dict_with_dev_and_prod_readonly_keys(client, monkeypatch):
    def mock_get_keys(_, __):
        return {
            "dev": "123456789",
            "prod": "123456789"
        }
    monkeypatch.setattr(ExampleClass, "_get_keys", mock_get_keys)
    assert client.read_only_keys == {
        "dev": "123456789",
        "prod": "123456789"
    }

• … and so on for the rest of the test functions
• In this example, we define a client fixture that returns an instance of the class.
• Then, we can pass this fixture as an argument to each test function that needs it.
• This way, the ExampleClass class is only instantiated once and reused across all test functions.
• In pytest, you can use fixtures to set up preconditions for your tests.
• Fixtures are defined using the @pytest.fixture decorator and can take parameters.
• Here’s a simpler example that demonstrates how to initialize a class with two arguments before running tests that check the values of its fields:

import pytest

class MyClass:
    def __init__(self, name, age):
        self.name = name
        self.age = age

@pytest.fixture
def my_class():
    return MyClass(name="pavol", age=39)

def test_name(my_class):
    assert my_class.name == "pavol"

def test_age(my_class):
    assert my_class.age == 39

• Please note that the following, a more intuitive approach of “class fixtures” is not supported (maybe in the future, the error message says:)

""" DOES NOT WORK """
@pytest.fixture
class AccessConfig:
    def __init__(self):
        self.client_name = "com_snowplowanalytics"
        self.env = "prod1"

# > ..\..\..\..# \AppData\Local\Programs\Python\Python311\Lib\site-packages\_pytest\fixtures.# py:1195: in __call__
#     raise ValueError("class fixtures not supported (maybe in the future)")
# E   ValueError: class fixtures not supported (maybe in the future)