Testing Flask Applications

Flask provides utilities for testing an application. This documentation goes over techniques for working with different parts of the application in tests.

We will use the pytest framework to set up and run our tests.

$ pip install pytest

The tutorial goes over how to write tests for 100% coverage of the sample Flaskr blog application. See the tutorial on tests for a detailed explanation of specific tests for an application.

Identifying Tests

Tests are typically located in the tests folder. Tests are functions that start with test_, in Python modules that start with test_. Tests can also be further grouped in classes that start with Test.

It can be difficult to know what to test. Generally, try to test the code that you write, not the code of libraries that you use, since they are already tested. Try to extract complex behaviors as separate functions to test individually.

Fixtures

Pytest fixtures allow writing pieces of code that are reusable across tests. A simple fixture returns a value, but a fixture can also do setup, yield a value, then do teardown. Fixtures for the application, test client, and CLI runner are shown below, they can be placed in tests/conftest.py.

If you’re using an application factory, define an app fixture to create and configure an app instance. You can add code before and after the yield to set up and tear down other resources, such as creating and clearing a database.

If you’re not using a factory, you already have an app object you can import and configure directly. You can still use an app fixture to set up and tear down resources.

import pytest
from my_project import create_app

@pytest.fixture()
def app():
    app = create_app()
    app.config.update({
        "TESTING": True,
    })

    # other setup can go here

    yield app

    # clean up / reset resources here


@pytest.fixture()
def client(app):
    return app.test_client()


@pytest.fixture()
def runner(app):
    return app.test_cli_runner()

Sending Requests with the Test Client

The test client makes requests to the application without running a live server. Flask’s client extends Werkzeug’s client, see those docs for additional information.

The client has methods that match the common HTTP request methods, such as client.get() and client.post(). They take many arguments for building the request; you can find the full documentation in EnvironBuilder. Typically you’ll use path, query, headers, and data or json.

To make a request, call the method the request should use with the path to the route to test. A TestResponse is returned to examine the response data. It has all the usual properties of a response object. You’ll usually look at response.data, which is the bytes returned by the view. If you want to use text, Werkzeug 2.1 provides response.text, or use response.get_data(as_text=True).

def test_request_example(client):
    response = client.get("/posts")
    assert b"<h2>Hello, World!</h2>" in response.data

Pass a dict query={"key": "value", ...} to set arguments in the query string (after the ? in the URL). Pass a dict headers={} to set request headers.

To send a request body in a POST or PUT request, pass a value to data. If raw bytes are passed, that exact body is used. Usually, you’ll pass a dict to set form data.

Form Data

To send form data, pass a dict to data. The Content-Type header will be set to multipart/form-data or application/x-www-form-urlencoded automatically.

If a value is a file object opened for reading bytes ("rb" mode), it will be treated as an uploaded file. To change the detected filename and content type, pass a (file, filename, content_type) tuple. File objects will be closed after making the request, so they do not need to use the usual with open() as f: pattern.

It can be useful to store files in a tests/resources folder, then use pathlib.Path to get files relative to the current test file.

from pathlib import Path

# get the resources folder in the tests folder
resources = Path(__file__).parent / "resources"

def test_edit_user(client):
    response = client.post("/user/2/edit", data={
        "name": "Flask",
        "theme": "dark",
        "picture": (resources / "picture.png").open("rb"),
    })
    assert response.status_code == 200

JSON Data

To send JSON data, pass an object to json. The Content-Type header will be set to application/json automatically.

Similarly, if the response contains JSON data, the response.json attribute will contain the deserialized object.

def test_json_data(client):
    response = client.post("/graphql", json={
        "query": """
            query User($id: String!) {
                user(id: $id) {
                    name
                    theme
                    picture_url
                }
            }
        """,
        variables={"id": 2},
    })
    assert response.json["data"]["user"]["name"] == "Flask"

Following Redirects

By default, the client does not make additional requests if the response is a redirect. By passing follow_redirects=True to a request method, the client will continue to make requests until a non-redirect response is returned.

TestResponse.history is a tuple of the responses that led up to the final response. Each response has a request attribute which records the request that produced that response.

def test_logout_redirect(client):
    response = client.get("/logout")
    # Check that there was one redirect response.
    assert len(response.history) == 1
    # Check that the second request was to the index page.
    assert response.request.path == "/index"

Accessing and Modifying the Session

To access Flask’s context variables, mainly session, use the client in a with statement. The app and request context will remain active after making a request, until the with block ends.

from flask import session

def test_access_session(client):
    with client:
        client.post("/auth/login", data={"username": "flask"})
        # session is still accessible
        assert session["user_id"] == 1

    # session is no longer accessible

If you want to access or set a value in the session before making a request, use the client’s session_transaction() method in a with statement. It returns a session object, and will save the session once the block ends.

from flask import session

def test_modify_session(client):
    with client.session_transaction() as session:
        # set a user id without going through the login route
        session["user_id"] = 1

    # session is saved now

    response = client.get("/users/me")
    assert response.json["username"] == "flask"

Running Commands with the CLI Runner

Flask provides test_cli_runner() to create a FlaskCliRunner, which runs CLI commands in isolation and captures the output in a Result object. Flask’s runner extends Click’s runner, see those docs for additional information.

Use the runner’s invoke() method to call commands in the same way they would be called with the flask command from the command line.

import click

@app.cli.command("hello")
@click.option("--name", default="World")
def hello_command(name):
    click.echo(f"Hello, {name}!")

def test_hello_command(runner):
    result = runner.invoke(args="hello")
    assert "World" in result.output

    result = runner.invoke(args=["hello", "--name", "Flask"])
    assert "Flask" in result.output

Tests that depend on an Active Context

You may have functions that are called from views or commands, that expect an active application context or request context because they access request, session, or current_app. Rather than testing them by making a request or invoking the command, you can create and activate a context directly.

Use with app.app_context() to push an application context. For example, database extensions usually require an active app context to make queries.

def test_db_post_model(app):
    with app.app_context():
        post = db.session.query(Post).get(1)

Use with app.test_request_context() to push a request context. It takes the same arguments as the test client’s request methods.

def test_validate_user_edit(app):
    with app.test_request_context(
        "/user/2/edit", method="POST", data={"name": ""}
    ):
        # call a function that accesses `request`
        messages = validate_edit_user()

    assert messages["name"][0] == "Name cannot be empty."

Creating a test request context doesn’t run any of the Flask dispatching code, so before_request functions are not called. If you need to call these, usually it’s better to make a full request instead. However, it’s possible to call them manually.

def test_auth_token(app):
    with app.test_request_context("/user/2/edit", headers={"X-Auth-Token": "1"}):
        app.preprocess_request()
        assert g.user.name == "Flask"