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_string
, 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_string={"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", follow_redirects=True)
# 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"