Before we go ahead learning how to create more complex web applications we need to learn another very important feature of Flask.

Flask makes it very easy to test your web application without even running a web server.

For this we created a file called test_app.py in the same folder as we have the app.py with the following content. The name of the files must start with the word test_, but otherwise you can pick any filename.

Inside we import the application and we have a test function, again its name must start with test_. From the app we can get the test_client which is a representation of our running web application.

Then we can send in various requests. In this case we sent in an HTTP GET request to the root of the site.

We get back a response object that we can then interrogate with various assertions.

To run this we'll need to install pytest and then we can just type in pytest. It will find and run the tests.

import app

def test_app():
    web = app.app.test_client()

    rv = web.get('/')
    assert rv.status == '200 OK'
    assert rv.data == b'Hello World!'

pytest