Avoiding test interdependence in PyTest

Introduction

Tests should work on their own without relying on other tests. This helps find problems faster and keeps tests reliable.

When writing multiple tests for a feature that share some setup

When fixing a bug that appears only when tests run together

When adding new tests to an existing test suite

When tests sometimes fail randomly without code changes

When running tests in parallel or on different machines

Syntax

PyTest

def test_example():
    # setup code here
    assert something == expected

# Use fixtures or setup functions to prepare test data independently

Each test should prepare its own data or use fixtures that reset state.

Avoid sharing data or state between tests unless reset properly.

Examples

A simple test that does not depend on anything else.

PyTest

def test_addition():
    result = 2 + 3
    assert result == 5

Using a fixture to provide fresh data for each test avoids interference.

PyTest

import pytest

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

def test_append(sample_list):
    sample_list.append(4)
    assert sample_list == [1, 2, 3, 4]

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

Sample Program

This example shows two tests using the same fixture. Each test gets a new empty list, so they don't affect each other.

PyTest

import pytest

@pytest.fixture
def fresh_list():
    return []

def test_add_item(fresh_list):
    fresh_list.append('item')
    assert fresh_list == ['item']

def test_empty_list(fresh_list):
    assert fresh_list == []

Important Notes

Use fixtures to create fresh test data for each test.

Do not rely on the order tests run in; tests should pass independently.

Reset any shared resources or states between tests.

Summary

Tests should not depend on each other to keep results clear and reliable.

Use fixtures or setup functions to prepare independent test data.

Always reset shared state to avoid hidden bugs.