Test independence in PyTest - Build an Automation Script

Verify that tests run independently without relying on each other's state

Preconditions (2)

Step 1: Run test_addition which adds two numbers and checks the result

Step 2: Run test_reset which resets the calculator state

Step 3: Run test_addition again to verify it does not depend on previous test state

✅ Expected Result: Each test passes independently, no test depends on the result or state of another test

Automation Requirements - pytest

Assertions Needed:

Assert addition result is correct

Assert calculator state is reset properly

Assert tests do not share state causing failures

Best Practices:

Use fixtures to provide fresh state for each test

Avoid sharing mutable state between tests

Keep tests isolated and independent

Automated Solution

PyTest

import pytest

class Calculator:
    def __init__(self):
        self.value = 0

    def add(self, x):
        self.value += x
        return self.value

    def reset(self):
        self.value = 0

@pytest.fixture

def calculator():
    return Calculator()


def test_addition(calculator):
    result = calculator.add(5)
    assert result == 5


def test_reset(calculator):
    calculator.add(10)
    calculator.reset()
    assert calculator.value == 0


def test_addition_again(calculator):
    result = calculator.add(3)
    assert result == 3

The Calculator class simulates a simple calculator with add and reset methods.

The calculator fixture creates a new Calculator instance for each test, ensuring fresh state.

test_addition adds 5 and checks the result is 5.

test_reset adds 10, resets, and checks the value is 0.

test_addition_again adds 3 and checks the result is 3, verifying no leftover state from previous tests.

This setup ensures tests are independent and do not affect each other.

Common Mistakes - 3 Pitfalls

{'mistake': 'Sharing a single Calculator instance across tests as a global variable', 'why_bad': "Tests can affect each other's state causing flaky or false failures", 'correct_approach': 'Use pytest fixtures to provide a fresh Calculator instance for each test'}

Modifying global or class-level variables inside tests

Relying on test execution order to pass tests

Bonus Challenge

Now add data-driven testing with 3 different addition inputs using pytest parametrize

Show Hint