.. _testing:

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 :mod:`unittest` package that comes pre-installed with Python.

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

First, we need an application to test; we will use the application from 
the :ref:`tutorial`.  If you don't have that application yet, get the 
sources from `the examples`_.

.. _the examples:
   http://github.com/mitsuhiko/flask/tree/master/examples/flaskr/

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

In order to test the application, we add a second module 
(`flaskr_tests.py`) and create a unittest skeleton there::

    import os
    import flaskr
    import unittest
    import tempfile

    class FlaskrTestCase(unittest.TestCase):

        def setUp(self):
            self.db_fd, flaskr.app.config['DATABASE'] = tempfile.mkstemp()
            flaskr.app.config['TESTING'] = True
            self.app = flaskr.app.test_client()
            flaskr.init_db()

        def tearDown(self):
            os.close(self.db_fd)
            os.unlink(flaskr.app.config['DATABASE'])

    if __name__ == '__main__':
        unittest.main()

The code in the :meth:`~unittest.TestCase.setUp` method creates a new test
client and initializes a new database.  This function is called before
each individual test function is run.  To delete the database after the 
test, we close the file and remove it from the filesystem in the
:meth:`~unittest.TestCase.tearDown` method.  Additionally during setup the
``TESTING`` config flag is activated.  What it does is disabling the error
catching during request handling so that you get better error reports when
performing test requests against the application.

This test client will give us a simple interface to the application.  We can 
trigger test requests to the application, and the client will also keep track 
of cookies for us.

Because SQLite3 is filesystem-based we can easily use the 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.

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

    $ python flaskr_tests.py

    ----------------------------------------------------------------------
    Ran 0 tests in 0.000s

    OK

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.

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 method to our class, like this::

    class FlaskrTestCase(unittest.TestCase):

        def setUp(self):
            self.db_fd, flaskr.app.config['DATABASE'] = tempfile.mkstemp()
            self.app = flaskr.app.test_client()
            flaskr.init_db()

        def tearDown(self):
            os.close(self.db_fd)
            os.unlink(flaskr.DATABASE)

        def test_empty_db(self):
            rv = self.app.get('/')
            assert 'No entries here so far' in rv.data

Notice that our test functions begin with the word `test`; this allows 
:mod:`unittest` to automatically identify the method as a test to run. 

By using `self.app.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.BaseResponse.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::

    $ python flaskr_tests.py
    .
    ----------------------------------------------------------------------
    Ran 1 test in 0.034s

    OK

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 methods to your `FlaskrTestCase` class::

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

   def logout(self):
       return self.app.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 to the class::

   def test_login_logout(self):
       rv = self.login('admin', 'default')
       assert 'You were logged in' in rv.data
       rv = self.logout()
       assert 'You were logged out' in rv.data
       rv = self.login('adminx', 'default')
       assert 'Invalid username' in rv.data
       rv = self.login('admin', 'defaultx')
       assert 'Invalid password' in rv.data

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

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

    def test_messages(self):
        self.login('admin', 'default')
        rv = self.app.post('/add', data=dict(
            title='<Hello>',
            text='<strong>HTML</strong> allowed here'
        ), follow_redirects=True)
        assert 'No entries here so far' not in rv.data
        assert '&lt;Hello&gt;' in rv.data
        assert '<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::

    $ python flaskr_tests.py
    ...
    ----------------------------------------------------------------------
    Ran 3 tests in 0.332s

    OK

For more complex tests with headers and status codes, check out the
`MiniTwit Example`_ from the sources which contains a larger test
suite.


.. _MiniTwit Example:
   http://github.com/mitsuhiko/flask/tree/master/examples/minitwit/


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::

    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 :ref:`app-factories`).

Note however that if you are using a test request context, the
:meth:`~flask.Flask.before_request` functions are not automatically called
same for :meth:`~flask.Flask.after_request` functions.  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.


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).
However, keep in mind that any :meth:`~flask.Flask.after_request` functions
are already called at this point so your database connection and
everything involved is probably already closed down.