Flask Testing

This repo demonstrates how to test a Flask application.

Flask in Production

When we launch a Flask application, the web client interacts with the server, which is made up of three main subsystems:

The following steps occur whenever there is a request from a client:

• The Flask Web Server converts the HTTP message into a Request object and passes it to the Flask Routing System.
• The Flask Routing System takes a Request object and calls the appropriate User Code.
• The User Code performs whatever task we want and returns a Result to the Flask Routing System.
• The Flask Routing System converts the Result into an appropriate Response Object and sends it to the Flask Web Server.
• The Flask Web Server takes the Response Object and coverts it into an HTTP message.

Flask in Testing

When we want to test a Flask application, we only need to test the User Code (It is the job of the Flask community to test the Flask Routing System and the Flask Web Server). To do this, we replace the Flask Web Server with a Flask Test Client:

• The Pytest Test puts the Flask app into testing mode and creates the Flask Test Client. Using this client, the Pytest Test calls the .get() method for the endpoint we want to test.
• The Flask Test Client creates a Request object and passes it to the Flask Routing System.
• The Flask Routing System takes a Request object and calls the appropriate User Code.
• The User Code performs whatever task we want and returns a Result to the Flask Routing System.
• The Flask Routing System converts the Result into an appropriate Response Object and sends it to the Flask Test Client.
• The Flask Test Client returns the Response Object to the the test
• Our test can now assert that the response is correct.

The Mechanics of Testing

• In app.py our Flask app is declared as a global variable named app
• In test_app.py we import app from app.py
• In the test_hello function we:
  • put the app into testing mode
  • get the test client
  • use the test client to call the .get() method
  • assert that the .data of the response is what we expect

How to Run These Tests

• Create a virtual environment

  python3 -m venv .venv
  

• Activate the virtual environment

  source .venv/bin/activate
  

• Install the required packages (flask and pytest)

  pip install -r requirements.txt
  

• Execute the tests

  pytest