PyTest Tutorial: Install, Write, and Run Python Tests with Confidence

What is PyTest?

PyTest is a powerful, open‑source testing framework that lets developers write concise, scalable test cases for Python projects—be it unit tests, integration tests, or end‑to‑end API checks. It supports parallel execution, fixture injection, parameterization, and rich reporting out of the box.

Why Choose PyTest?

• Simple, readable syntax that reduces boilerplate
• Automatic test discovery and execution
• Run specific tests or groups with -k and markers
• Built‑in support for parallel runs via pytest-xdist
• Extensible through fixtures, hooks, and plugins
• Free and actively maintained by the community

Installing PyTest

Install the latest stable release with pip:

pip install pytest

Verify the installation:

pytest --version

For specific version control, you can pin to a release, e.g., pip install pytest==7.4.0.

Getting Started: A Basic Test

Create a project directory and add a test file named test_sample1.py:

import pytest

def test_file1_method1():
    x = 5
    y = 6
    assert x + 1 == y, "Addition failed"
    assert x == y, "Values should match"

def test_file1_method2():
    x = 5
    y = 6
    assert x + 1 == y, "Addition failed"

Run the test suite:

pytest

Output example:

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

test_sample1.py F.                                               [100%]

=================================== FAILURES ===================================
...

Understanding Assertions

PyTest assertions behave like standard Python assert statements. On failure, the test stops executing and PyTest reports the exact location and message:

assert "hello" == "Hai", "Mismatched greeting"

Test Discovery Rules

• Files must begin with test_ or end with _test.py.
• Test functions must start with test_.
• Other files or functions are ignored unless explicitly specified.

Running Specific Tests

Execute all tests in a directory:

pytest

Run a single file:

pytest test_sample1.py

Filter by substring:

pytest -k method1 -v

Run by marker:

pytest -m set1

Parallel Execution

Install the xdist plugin and launch a 4‑worker parallel run:

pip install pytest-xdist
pytest -n 4

Using Fixtures

Define reusable setup logic once and inject it into tests:

import pytest

@pytest.fixture
def supply_data():
    return [25, 35, 45]

def test_compare_with_first(supply_data):
    assert supply_data[0] == 25

Place fixture code in conftest.py to share across multiple test modules.

Parameterized Tests

Run a single test with many inputs:

import pytest

@pytest.mark.parametrize("a, b, expected", [(5, 5, 10), (3, 5, 8)])
def test_add(a, b, expected):
    assert a + b == expected

Skipping and Expected Failures

Use markers to skip or expect failures without polluting the results:

import pytest

@pytest.mark.skip(reason="Feature not yet available")
def test_future_feature():
    pass

@pytest.mark.xfail(reason="Known bug")
def test_known_issue():
    assert 1 == 2

Generating XML Reports

Generate a JUnit‑compatible XML file for CI pipelines:

pytest -v --junitxml=result.xml

Practical Example: Testing a Public API

Below is a minimal framework that exercises https://reqres.in/:

# conftest.py
import pytest

@pytest.fixture
def base_url():
    return "https://reqres.in/api"

# test_list_user.py
import requests, json
import pytest

@pytest.mark.parametrize("user_id, first_name", [(1, "George"), (2, "Janet")])
def test_valid_user(base_url, user_id, first_name):
    resp = requests.get(f"{base_url}/users/{user_id}")
    data = json.loads(resp.text)
    assert resp.status_code == 200
    assert data["data"]["id"] == user_id
    assert data["data"]["first_name"] == first_name


def test_invalid_user(base_url):
    resp = requests.get(f"{base_url}/users/50")
    assert resp.status_code == 404

# test_login_user.py
import requests, json
import pytest

def test_login_success(base_url):
    payload = {"email": "test@test.com", "password": "something"}
    resp = requests.post(f"{base_url}/login/", data=payload)
    data = json.loads(resp.text)
    assert resp.status_code == 200
    assert data["token"] == "QpwL5tke4Pnpja7X"


def test_login_missing_password(base_url):
    payload = {"email": "test@test.com"}
    resp = requests.post(f"{base_url}/login/", data=payload)
    data = json.loads(resp.text)
    assert resp.status_code == 400
    assert data["error"] == "Missing password"

Conclusion

This tutorial covered installing PyTest, writing clear tests, using fixtures and markers, running tests in parallel, generating XML reports, and building a simple API test suite. With these foundations, you can scale your Python testing practices confidently.