This content originally appeared on DEV Community and was authored by Gábor Szabó
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.
In order to do 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 using import app and we add a test function. Its name must start with test_, but otherwise we can pick any name.
From the app object we imported, that is, from our Flask application, 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 using web.get('/').
We get back a response object that we can then interrogate with various assertions.
To run this we'll need to install pytest:
pip install pytest
Then we can just type in pytest. It will find and run the tests.
pytest
The test file
import app
def test_app():
web = app.app.test_client()
rv = web.get('/')
assert rv.status == '200 OK'
assert rv.data == b'Hello World!'
This content originally appeared on DEV Community and was authored by Gábor Szabó
Gábor Szabó | Sciencx (2025-08-13T13:14:00+00:00) Flask: testing hello world. Retrieved from https://www.scien.cx/2025/08/13/flask-testing-hello-world/
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