How to do Unit testing in Python Flask application

You are currently viewing How to do Unit testing in Python Flask application


Testing is an essential aspect of software development that ensures the functionality and reliability of a program. One popular method of testing is unit testing, which evaluates individual components or “units” of an application to ensure they are working as intended. When dealing with web applications in Flask, a popular micro web framework written in Python, there are unique considerations and strategies for unit testing. In this blog, we will discuss how to perform unit testing in a Python Flask application.

Understanding Flask

Flask is a lightweight web framework for Python that provides a simple way to create web applications. It’s based on two main components: Werkzeug (a WSGI utility library) and Jinja (a template engine). Flask is often used for small projects or as a prototype for larger ones due to its ease of use and flexibility. For testing, Flask provides a test client to send requests to the application and check the responses.

What is Unit Testing?

Unit testing involves testing the smallest possible parts (units) of an application in isolation. It’s an approach that facilitates easy debugging and provides a reliable way to ensure that changes in code do not break the existing functionality. This testing method has proven to be incredibly beneficial in complex projects where changes are frequent and the impact on other parts of the codebase must be minimized.

Why is Unit Testing Important in Flask Applications?

Unit testing in Flask applications is essential for several reasons:

  1. Ensures Functionality: It verifies that each part of the application performs as expected.
  2. Facilitates Refactoring: Tests provide a safety net that allows developers to make changes without fear of breaking existing functionality.
  3. Improves Code Quality: Writing tests often leads to better-designed code because it encourages developers to write code that is easy to test, and thus, often more modular and reusable.
  4. Documentation: Well-written tests can serve as examples of how a piece of code is supposed to work, serving as useful documentation for developers.

Setting up the Environment

Before we dive into writing tests, let’s set up our environment. For our testing, we will use Python’s built-in unittest module. Here is a simple Flask application that we will be testing:

from flask import Flask, jsonify

app = Flask(__name__)

def hello_world():
    return jsonify(message="Hello, World!")

if __name__ == '__main__':

Getting Started with Unit Testing in Flask

Python’s unittest module provides a rich set of tools for constructing and running tests. To use it, you first need to import it into your test file, then create a new class that inherits from unittest.TestCase. Let’s create a new file

import unittest
from app import app

class BasicTestCase(unittest.TestCase):
    def test_home(self):
        tester = app.test_client(self)
        response = tester.get('/')
        self.assertEqual(response.status_code, 200)

if __name__ == '__main__':

Here, we’re defining a class BasicTestCase that contains our test methods. Each method in this class represents an individual unit test. The app.test_client(self) function is used to create a test client instance that we can use to simulate HTTP requests to our application. The tester.get('/') function sends a GET request to the specified URL and returns the server’s response.

In the test_home method, we’re testing that a GET request to the ‘/’ URL returns a 200 status code, indicating that the request was successful. If the actual response status code doesn’t match the expected status code (200), the self.assertEqual assertion will fail, and the test will fail.

More Advanced Testing

While our basic unit test is a good start, real-world applications often have more complex behavior that we need to test. Here are some examples of more advanced testing strategies:

Testing JSON data

Let’s say we want to test the JSON data returned by our application. We can do this using the get_json method of the response object:

def test_home_message(self):
    tester = app.test_client(self)
    response = tester.get('/')
    self.assertEqual(response.get_json(), {'message': 'Hello, World!'})

Here, we’re testing that the JSON data returned by the server matches the expected data. If the actual data doesn’t match the expected data, the test will fail.

Testing Error Handlers

Flask provides a way to handle errors that occur in your application using error handlers. You can test these error handlers to ensure they behave as expected. Let’s add a 404 error handler to our application:

def page_not_found(e):
    return jsonify(error="Page not found"), 404

Now, let’s write a test for this error handler:

def test_page_not_found(self):
    tester = app.test_client(self)
    response = tester.get('/non_existent_page')
    self.assertEqual(response.status_code, 404)
    self.assertEqual(response.get_json(), {'error': 'Page not found'})

In this test, we’re simulating a request to a non-existent page to trigger a 404 error. We’re then testing that the server responds with a 404 status code and the expected error message.

Testing Database Interaction

If your application interacts with a database, you’ll likely want to test this functionality as well. Flask provides a way to create and work with a test database for this purpose. Here’s an example of how you might set up a test database:

class DatabaseTestCase(unittest.TestCase):
    def setUp(self):
        app.config['TESTING'] = True
        app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:' = app.test_client()

    def tearDown(self):

    def test_database_functionality(self):
        # Your test code goes here

The setUp the method is a special method that gets called before each test is run. Here, we’re setting up a new test client and a new in-memory SQLite database.

The tearDown the method is a special method that gets called after each test is run. Here, we’re cleaning up the database to ensure that each test starts with a clean slate.


Unit testing is a crucial part of developing a Flask application, or any application for that matter. By breaking your application down into smaller, more manageable units and testing each one individually, you can greatly improve the reliability and maintainability of your application.

Remember, the key to effective unit testing is to write tests that are small, isolated, and deterministic. Each test should focus on a single behavior or functionality and should not depend on the results of other tests.

Frequently Asked Questions (FAQ)

1. What is Flask’s testing philosophy?

Flask follows the philosophy of ensuring that unit tests do not depend on external factors and should be isolated. This means that each test should run in its own environment and set up its own data.

2. What other testing tools can I use with Flask?

Python has a rich ecosystem of testing libraries that you can use with Flask. These include pytest for more advanced features like fixtures, hypothesis for property-based testing, and mock for mocking objects.

3. How do I run my tests?

You can run your tests using the command python -m unittest test_app from the terminal. You can also use a tool like pytest to run your tests with the command pytest.

4. How can I test views that require authentication?

You can simulate a logged-in user by setting the session object in your test. For example, if your application uses a user’s ID to track their login status, you could set session['user_id'] it to simulate a logged-in user.

5. How do I write tests for forms in my Flask application?

To write tests for forms, you can use the test_client object to send POST requests to your application. You can include the form data in the body of the request. Remember to also include any necessary CSRF tokens in your request.

6. Can I perform Integration testing with Flask?

Yes, Flask’s test client can be used to simulate requests to your application, making it suitable for both unit tests and integration tests.

7. How can I achieve full coverage in Flask?

To achieve full coverage, you should aim to write a test for every route in your application. For each route, test all possible branches of your code (such as if-else statements). You can also use a tool like to measure your test coverage.

8. What should I do when a test fails?

When a test fails, you should first look at the error message to understand what went wrong. The error message will usually tell you what assertion failed and why. From there, you can debug the problem by looking at the code being tested and the test itself.

Atiqur Rahman

I am MD. Atiqur Rahman graduated from BUET and is an AWS-certified solutions architect. I have successfully achieved 6 certifications from AWS including Cloud Practitioner, Solutions Architect, SysOps Administrator, and Developer Associate. I have more than 8 years of working experience as a DevOps engineer designing complex SAAS applications.

Leave a Reply