Pytest is a popular Python library used for testing. It is my preferred testing library because it requires less boilerplate code than the alternatives such as (the builtin) unittest, the built in testing library. In this article, I will show you how you can use pytest-flask and pytest-mock to test your Flask app. These two libraries are plugins for Pytest which build upon some of the features that Pytest provides us.

In this example, we will be testing a very simple Flask app I created source code here. If you want to learn more about the Flask app you can read my previous article here :plug: :plug:. All you really need to know for this article is that it involves using a very simple RESTful API for an imaginary cat store. Essentially all the API does is it interacts with a database, to get current cats, add new cats, edit already existing cats and remove cats from the store.

Prerequisites

  • Your own Flask app
  • Install the following dependencies, using pip install -r requirements.txt (or pip3 instead of pip)

Where requirements.txt is:

attrs==19.3.0
click==7.1.2
Flask==1.1.2
Flask-SQLAlchemy==2.4.3
itsdangerous==1.1.0
Jinja2==2.11.2
MarkupSafe==1.1.1
more-itertools==8.4.0
packaging==20.4
pluggy==0.13.1
psycopg2==2.8.5
py==1.8.2
pyparsing==2.4.7
pytest==5.4.3
pytest-flask==1.0.0
pytest-mock==3.1.1
six==1.15.0
SQLAlchemy==1.3.17
wcwidth==0.2.4
Werkzeug==1.0.1

Libraries

pytest-flask: Allows us to specify an app fixture and then send API requests with this app. Usage is similar to requests library, when sending HTTP requests to our flask app.

pytest-mock: Is a simple wrapper around the unittest mock library, so anything you can do using unittest.mock you can do with pytest-mock. The main difference in usage is you can access it using a fixture mocker, also the mock ends at the end of the test. Whereas with the normal mock library if you mock say the open() function, it will be mocked for the remaining duration of that test module, i.e. it will effect other tests.

pytest-flask example

import pytest

from example_app import create_app


@pytest.fixture
def app():
    app = create_app()
    return app


def test_example(client):
    response = client.get("/")
    assert response.status_code == 200

To use pytest-flask we need to create a fixture called app() which creates our Flask server. We can then use this fixture by passing client as an argument to any test. Then we can send various http requests using client.

Above is a very simple example using pytest-flask, we send a GET request to our app, which should return all cats in the database. We then check that the status code returned from the server was a 200 (OK). This is great except how can we mock out certain features within our code?

pytest-mock

We can mock out certain parts of our code using the pytest-mock library, but we have to mock inside the app() fixture. Since the rest of our tests will just be making HTTP requests to our Flask server. In this example say we don’t want to mock a connection to the database, we can use the following lines of code.

mocker.patch("flask_sqlalchemy.SQLAlchemy.init_app", return_value=True)
mocker.patch("flask_sqlalchemy.SQLAlchemy.create_all", return_value=True)
mocker.patch("example.database.get_all", return_value={})

What this bit of code is doing is any time the any of the mocked functions are called, say init_app we mock it so it will always return true. We have to give it the “full path” to the function, it’s the same as if you had to import the function itself.

import pytest

import example.app


@pytest.fixture
def app(mocker):
    mocker.patch("flask_sqlalchemy.SQLAlchemy.init_app", return_value=True)
    mocker.patch("flask_sqlalchemy.SQLAlchemy.create_all", return_value=True)
    mocker.patch("example.database.get_all", return_value={})
    return example.app.app


def test_example(client):
    response = client.get("/")
    assert response.status_code == 200

In this example it’s very boring as when we send an HTTP GET request to the app it will not interact with the database since we’ve mocked this out but instead just return an empty dict ({}). In reality, this is not a very good test, you would make it a bit more interesting.

virtualenv .venv
source .venv/bin/activate
pip install -e .
pip install -r requirements.txt
export $(xargs < database.conf)
pytest

Appendix