App Development Resources

Session Objective

To explore the essential features of Pytest in Python including fixtures, markers, parameterization, and organizing reusable code using conftest.py. By the end, students will understand how to write clear and maintainable tests using idiomatic Pytest practices.

Introduction to Pytest

Pytest is a testing framework that makes it easy to write simple and scalable test code.
Install: pip install pytest
Test file format: test_*.py or *_test.py
Test function naming: must start with test_

Example:

def func(x):
    return x + 1

def test_answer():
    assert func(3) == 4

Run using: pytest file.py

Fixtures

Fixtures are functions that run before (and sometimes after) tests. They help set up test context.

Analogy: Think of a fixture like a butler who sets your table before every meal (test).

Basic Fixture:

import pytest

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

def test_sum(sample_list):
    assert sum(sample_list) == 6

Run using: pytest file.py

Scope:

function (default): New fixture for each test.
module, class, session: Controls how often the fixture is run.

@pytest.fixture(scope="module")
def connect_db():
    print("Connecting once for module")
    yield "db_connection"
    print("Disconnecting after module")

Parameterization

Allows you to run the same test with different inputs.

Example:

import pytest

@pytest.mark.parametrize("a,b,result", [(1, 2, 3), (3, 4, 7), (5, 5, 10)])
def test_addition(a, b, result):
    assert a + b == result

Analogy: Like giving a quiz to multiple students with the same question but different answers.

Skipping & Expected Failures

import pytest

@pytest.mark.skip(reason="Work in progress")
def test_login():
    assert False

@pytest.mark.xfail(reason="Bug #101")
def test_feature():
    assert 1 == 2

Usage:

@pytest.mark.skip: Skip a test
@pytest.mark.xfail: Expect it to fail (without breaking suite)

Grouping with Markers & Filtering

Markers allow grouping tests and selectively running them.

Custom Marker:

import pytest

@pytest.mark.slow
def test_large_data():
    assert True

Run only slow tests: pytest -m slow

You can also use keyword filters:

pytest -k "addition"

Shared Fixtures with conftest.py

Why? To avoid duplicating fixtures across test files.

Analogy: Like placing shared utensils in a kitchen drawer (conftest.py) instead of repeating them at every table.

Structure:

project/
├── tests/
│   ├── test_math.py
│   ├── test_string.py
│   └── conftest.py

conftest.py:

import pytest

@pytest.fixture
def fruit_basket():
    return ["apple", "banana", "cherry"]

test_math.py:

def test_banana_in_basket(fruit_basket):
    assert "banana" in fruit_basket

Pytest automatically looks for fixtures in conftest.py
No need to import fruit_basket manually

Post-Session Practice Resources

Unit Testing in Flask Using Pytest

Unit testing is a core practice in Flask application development, allowing you to validate individual parts (units) of your application to ensure they behave as expected. pytest is a popular Python testing framework that works seamlessly with Flask.

1. Setting Up the Project

Make sure you have Flask and pytest installed:

pip install Flask pytest

Project Structure:

myapp/
├── app.py
├── test_app.py

2. Minimal Flask App to Test

app.py

from flask import Flask

app = Flask(__name__)

@app.route('/')
def index():
    return 'Hello, World!'

@app.route('/about')
def about():
    return 'This is the about page.'

@app.route('/greet/<name>')
def greet(name):
    return f'Hello, {name}!'

3. Writing Tests with Pytest

test_app.py

import pytest
from app import app

@pytest.fixture
def client():
    app.config['TESTING'] = True
    with app.test_client() as client:
        yield client

def test_index(client):
    response = client.get('/')
    assert response.status_code == 200
    assert b'Hello, World!' in response.data

def test_about(client):
    response = client.get('/about')
    assert response.status_code == 200
    assert b'This is the about page.' in response.data

def test_greet(client):
    response = client.get('/greet/Alice')
    assert response.status_code == 200
    assert b'Hello, Alice!' in response.data

4. Key Concepts Explained with Code

Using `test_` Prefix

All test functions must start with test_ for pytest to detect them.

Using Fixtures

The client() fixture provides a fresh Flask test client for every test, which simulates browser-like behavior.

Testing Routes

client.get('/some-url') simulates a GET request.
Use assert to check status codes and response content.

Byte Strings

Flask’s response.data is in bytes, so compare using b'Text'.

5. Running the Tests

Navigate to the directory containing test_app.py and run:

pytest

You’ll see output like:

================== test session starts ==================
collected 3 items

test_app.py ...                                        [100%]

=================== 3 passed in 0.20s ===================

6. Tips

Group related tests in classes using class TestViews: or class TestAuth:
Use separate files like test_models.py, test_forms.py, etc. for modularity.
Use temporary databases and mock data for more complex tests.

Summary

Minimal setup: Flask + Pytest
Use test_ prefix for all test functions
Simulate requests using client.get() / client.post()
Validate response with assert

Bonus Reference Table: Testing Types and Focus Areas

Test Type	Focus Area
Unit Testing	Smallest logical units (functions/methods)
Integration Testing	Multiple components/modules working together
System Testing	Whole app in full environment including server
System Testing Automation	Simulated end-user browser-based interaction
User Acceptance Testing	Real-world verification by actual users before release