Testing Flask Applications
==========================

   **Something that is untested is broken.**

The origin of this quote is unknown and while it is not entirely correct, it
is also not far from the truth.  Untested applications make it hard to
improve existing code and developers of untested applications tend to
become pretty paranoid.  If an application has automated tests, you can
safely make changes and instantly know if anything breaks.

Flask provides a way to test your application by exposing the Werkzeug
test :class:`~werkzeug.test.Client` and handling the context locals for you.
You can then use that with your favourite testing solution.

In this documentation we will use the `pytest`_ package as the base
framework for our tests. You can install it with ``pip``, like so::

    $ pip install pytest

.. _pytest: https://docs.pytest.org/

The Application
---------------

First, we need an application to test; we will use the application from
the :doc:`tutorial/index`. If you don't have that application yet, get
the source code from :gh:`the examples <examples/tutorial>`.

So that we can import the module ``flaskr`` correctly, we need to run
``pip install -e .`` in the folder ``tutorial``.

The Testing Skeleton
--------------------

We begin by adding a tests directory under the application root.  Then
create a Python file to store our tests (:file:`test_flaskr.py`). When we
format the filename like ``test_*.py``, it will be auto-discoverable by
pytest.

Next, we create a `pytest fixture`_ called
:func:`client` that configures
the application for testing and initializes a new database::

    import os
    import tempfile

    import pytest

    from flaskr import create_app


    @pytest.fixture
    def client():
        db_fd, flaskr.app.config['DATABASE'] = tempfile.mkstemp()
        flaskr.app.config['TESTING'] = True

        with flaskr.app.test_client() as client:
            with flaskr.app.app_context():
                flaskr.init_db()
            yield client

        os.close(db_fd)
        os.unlink(flaskr.app.config['DATABASE'])

This client fixture will be called by each individual test.  It gives us a
simple interface to the application, where we can trigger test requests to the
application.  The client will also keep track of cookies for us.

During setup, the ``TESTING`` config flag is activated.  What
this does is disable error catching during request handling, so that
you get better error reports when performing test requests against the
application.

Because SQLite3 is filesystem-based, we can easily use the
:mod:`tempfile` module to create a temporary database and initialize it.
The :func:`~tempfile.mkstemp` function does two things for us: it returns a
low-level file handle and a random file name, the latter we use as
database name.  We just have to keep the `db_fd` around so that we can use
the :func:`os.close` function to close the file.

To delete the database after the test, the fixture closes the file and removes
it from the filesystem.

If we now run the test suite, we should see the following output::

    $ pytest

    ================ test session starts ================
    rootdir: ./flask/examples/flaskr, inifile: setup.cfg
    collected 0 items

    =========== no tests ran in 0.07 seconds ============

Even though it did not run any actual tests, we already know that our
``flaskr`` application is syntactically valid, otherwise the import
would have died with an exception.

.. _pytest fixture:
   https://docs.pytest.org/en/latest/fixture.html

The First Test
--------------

Now it's time to start testing the functionality of the application.
Let's check that the application shows "No entries here so far" if we
access the root of the application (``/``).  To do this, we add a new
test function to :file:`test_flaskr.py`, like this::

    def test_empty_db(client):
        """Start with a blank database."""

        rv = client.get('/')
        assert b'No entries here so far' in rv.data

Notice that our test functions begin with the word `test`; this allows
`pytest`_ to automatically identify the function as a test to run.

By using ``client.get`` we can send an HTTP ``GET`` request to the
application with the given path.  The return value will be a
:class:`~flask.Flask.response_class` object. We can now use the
:attr:`~werkzeug.wrappers.Response.data` attribute to inspect
the return value (as string) from the application.
In this case, we ensure that ``'No entries here so far'``
is part of the output.

Run it again and you should see one passing test::

    $ pytest -v

    ================ test session starts ================
    rootdir: ./flask/examples/flaskr, inifile: setup.cfg
    collected 1 items

    tests/test_flaskr.py::test_empty_db PASSED

    ============= 1 passed in 0.10 seconds ==============

Logging In and Out
------------------

The majority of the functionality of our application is only available for
the administrative user, so we need a way to log our test client in and out
of the application.  To do this, we fire some requests to the login and logout
pages with the required form data (username and password).  And because the
login and logout pages redirect, we tell the client to `follow_redirects`.

Add the following two functions to your :file:`test_flaskr.py` file::

    def login(client, username, password):
        return client.post('/login', data=dict(
            username=username,
            password=password
        ), follow_redirects=True)


    def logout(client):
        return client.get('/logout', follow_redirects=True)

Now we can easily test that logging in and out works and that it fails with
invalid credentials.  Add this new test function::

    def test_login_logout(client):
        """Make sure login and logout works."""

        username = flaskr.app.config["USERNAME"]
        password = flaskr.app.config["PASSWORD"]

        rv = login(client, username, password)
        assert b'You were logged in' in rv.data

        rv = logout(client)
        assert b'You were logged out' in rv.data

        rv = login(client, f"{username}x", password)
        assert b'Invalid username' in rv.data

        rv = login(client, username, f'{password}x')
        assert b'Invalid password' in rv.data

Test Adding Messages
--------------------

We should also test that adding messages works.  Add a new test function
like this::

    def test_messages(client):
        """Test that messages work."""

        login(client, flaskr.app.config['USERNAME'], flaskr.app.config['PASSWORD'])
        rv = client.post('/add', data=dict(
            title='<Hello>',
            text='<strong>HTML</strong> allowed here'
        ), follow_redirects=True)
        assert b'No entries here so far' not in rv.data
        assert b'&lt;Hello&gt;' in rv.data
        assert b'<strong>HTML</strong> allowed here' in rv.data

Here we check that HTML is allowed in the text but not in the title,
which is the intended behavior.

Running that should now give us three passing tests::

    $ pytest -v

    ================ test session starts ================
    rootdir: ./flask/examples/flaskr, inifile: setup.cfg
    collected 3 items

    tests/test_flaskr.py::test_empty_db PASSED
    tests/test_flaskr.py::test_login_logout PASSED
    tests/test_flaskr.py::test_messages PASSED

    ============= 3 passed in 0.23 seconds ==============


Other Testing Tricks
--------------------

Besides using the test client as shown above, there is also the
:meth:`~flask.Flask.test_request_context` method that can be used
in combination with the ``with`` statement to activate a request context
temporarily.  With this you can access the :class:`~flask.request`,
:class:`~flask.g` and :class:`~flask.session` objects like in view
functions.  Here is a full example that demonstrates this approach::

    import flask

    app = flask.Flask(__name__)

    with app.test_request_context('/?name=Peter'):
        assert flask.request.path == '/'
        assert flask.request.args['name'] == 'Peter'

All the other objects that are context bound can be used in the same
way.

If you want to test your application with different configurations and
there does not seem to be a good way to do that, consider switching to
application factories (see :doc:`patterns/appfactories`).

Note however that if you are using a test request context, the
:meth:`~flask.Flask.before_request` and :meth:`~flask.Flask.after_request`
functions are not called automatically.  However
:meth:`~flask.Flask.teardown_request` functions are indeed executed when
the test request context leaves the ``with`` block.  If you do want the
:meth:`~flask.Flask.before_request` functions to be called as well, you
need to call :meth:`~flask.Flask.preprocess_request` yourself::

    app = flask.Flask(__name__)

    with app.test_request_context('/?name=Peter'):
        app.preprocess_request()
        ...

This can be necessary to open database connections or something similar
depending on how your application was designed.

If you want to call the :meth:`~flask.Flask.after_request` functions you
need to call into :meth:`~flask.Flask.process_response` which however
requires that you pass it a response object::

    app = flask.Flask(__name__)

    with app.test_request_context('/?name=Peter'):
        resp = Response('...')
        resp = app.process_response(resp)
        ...

This in general is less useful because at that point you can directly
start using the test client.

.. _faking-resources:

Faking Resources and Context
----------------------------

.. versionadded:: 0.10

A very common pattern is to store user authorization information and
database connections on the application context or the :attr:`flask.g`
object.  The general pattern for this is to put the object on there on
first usage and then to remove it on a teardown.  Imagine for instance
this code to get the current user::

    def get_user():
        user = getattr(g, 'user', None)
        if user is None:
            user = fetch_current_user_from_database()
            g.user = user
        return user

For a test it would be nice to override this user from the outside without
having to change some code.  This can be accomplished with
hooking the :data:`flask.appcontext_pushed` signal::

    from contextlib import contextmanager
    from flask import appcontext_pushed, g

    @contextmanager
    def user_set(app, user):
        def handler(sender, **kwargs):
            g.user = user
        with appcontext_pushed.connected_to(handler, app):
            yield

And then to use it::

    from flask import json, jsonify

    @app.route('/users/me')
    def users_me():
        return jsonify(username=g.user.username)

    with user_set(app, my_user):
        with app.test_client() as c:
            resp = c.get('/users/me')
            data = json.loads(resp.data)
            assert data['username'] == my_user.username


Keeping the Context Around
--------------------------

.. versionadded:: 0.4

Sometimes it is helpful to trigger a regular request but still keep the
context around for a little longer so that additional introspection can
happen.  With Flask 0.4 this is possible by using the
:meth:`~flask.Flask.test_client` with a ``with`` block::

    app = flask.Flask(__name__)

    with app.test_client() as c:
        rv = c.get('/?tequila=42')
        assert request.args['tequila'] == '42'

If you were to use just the :meth:`~flask.Flask.test_client` without
the ``with`` block, the ``assert`` would fail with an error because `request`
is no longer available (because you are trying to use it
outside of the actual request).


Accessing and Modifying Sessions
--------------------------------

.. versionadded:: 0.8

Sometimes it can be very helpful to access or modify the sessions from the
test client.  Generally there are two ways for this.  If you just want to
ensure that a session has certain keys set to certain values you can just
keep the context around and access :data:`flask.session`::

    with app.test_client() as c:
        rv = c.get('/')
        assert flask.session['foo'] == 42

This however does not make it possible to also modify the session or to
access the session before a request was fired.  Starting with Flask 0.8 we
provide a so called “session transaction” which simulates the appropriate
calls to open a session in the context of the test client and to modify
it. At the end of the transaction the session is stored and ready to be
used by the test client. This works independently of the session backend used::

    with app.test_client() as c:
        with c.session_transaction() as sess:
            sess['a_key'] = 'a value'

        # once this is reached the session was stored and ready to be used by the client
        c.get(...)

Note that in this case you have to use the ``sess`` object instead of the
:data:`flask.session` proxy.  The object however itself will provide the
same interface.


Testing JSON APIs
-----------------

.. versionadded:: 1.0

Flask has great support for JSON, and is a popular choice for building JSON
APIs. Making requests with JSON data and examining JSON data in responses is
very convenient::

    from flask import request, jsonify

    @app.route('/api/auth')
    def auth():
        json_data = request.get_json()
        email = json_data['email']
        password = json_data['password']
        return jsonify(token=generate_token(email, password))

    with app.test_client() as c:
        rv = c.post('/api/auth', json={
            'email': 'flask@example.com', 'password': 'secret'
        })
        json_data = rv.get_json()
        assert verify_token(email, json_data['token'])

Passing the ``json`` argument in the test client methods sets the request data
to the JSON-serialized object and sets the content type to
``application/json``. You can get the JSON data from the request or response
with ``get_json``.


.. _testing-cli:

Testing CLI Commands
--------------------

Click comes with `utilities for testing`_ your CLI commands. A
:class:`~click.testing.CliRunner` runs commands in isolation and
captures the output in a :class:`~click.testing.Result` object.

Flask provides :meth:`~flask.Flask.test_cli_runner` to create a
:class:`~flask.testing.FlaskCliRunner` that passes the Flask app to the
CLI automatically. Use its :meth:`~flask.testing.FlaskCliRunner.invoke`
method to call commands in the same way they would be called 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():
        runner = app.test_cli_runner()

        # invoke the command directly
        result = runner.invoke(hello_command, ['--name', 'Flask'])
        assert 'Hello, Flask' in result.output

        # or by name
        result = runner.invoke(args=['hello'])
        assert 'World' in result.output

In the example above, invoking the command by name is useful because it
verifies that the command was correctly registered with the app.

If you want to test how your command parses parameters, without running
the command, use its :meth:`~click.BaseCommand.make_context` method.
This is useful for testing complex validation rules and custom types. ::

    def upper(ctx, param, value):
        if value is not None:
            return value.upper()

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

    def test_hello_params():
        context = hello_command.make_context('hello', ['--name', 'flask'])
        assert context.params['name'] == 'FLASK'

.. _click: https://click.palletsprojects.com/
.. _utilities for testing: https://click.palletsprojects.com/testing/
