Deploying and Maintaining a Web App Part 3: Adding Tests with Pytest

Testing is an important part of development, including developing web applications. Among other benefits, they increase your confidence that your web application is doing what you expect, and a provide a basis for preventing bugs in an automated way when you make changes to your code.

Since testing is such an integral part of web application maintenance and deployment, in this Part 3 of our app-deploy project we’ll put in some basic tests to see how they can be implemented in flask applications and so we will eventually see how tests fit into the deployment workflow. We’ll be using the pytest python library as our testing framework.

To follow along, you can clone the repository:

git clone

And then go to the appropriate location in the code with the following command which includes all of the changes made in this post:

git checkout ed2ed02f8b3db

Finally, create the python environment with:

conda env create -f environment.yml

First, a Slight Upgrade to our App

Right now, when out user enters information into the app’s simple web form, the data is simply added to the database and nothing changes from the user’s perspective. To make testing a bit more interesting, we first add a little bit of additional functionality: our app will now print out a list of all the existing names in the database on the main page.

First we add a few lines to app/ so that it queries the database to get all names, and then sends the list of names to the template.

from flask import Flask, render_template, Blueprint
from .forms import NameForm
from .models import Name
from . import db
bp = Blueprint('app', __name__)
@bp.route('/', methods=['GET', 'POST'])
def home():
form = NameForm()
if form.validate_on_submit():
names = Name.query.all()
return render_template('index.html', form=form, names=names)
names = Name.query.all()
return render_template('index.html', form=form, names=names)
if __name__ == '__main__':

Then, we update the template to print out these names.

<h1>Hello World!</h1>
<h3>Please enter a name:</h3>
<form method=post>
{{ form.csrf_token }}
{{ }}
{{ form.submit }}
{% for name in names %}
{{ }}
{% endfor %}

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Now, when you enter in new names, they are listed out for you, like so:

Pytest Installation and Set-up

To start off, let’s activate our app’s conda environment and install pytest:

source activate app-deploy
conda install pytest

Our tests will be stored in a top level directory in a folder that we’ll call `tests`. Our tests will be stored in *.py files within this folder. These files must either be named like test_*.py or * this is how you tell pytest that these files represent tests (in other words, this is how pytest does “test discovery”).

As a reminder, your top level directory should look something like this:

So, let’s add a file called in /test.

touch test/

To run your tests, you simply just have to enter ‘pytest’ in the command line as follows. No tests have been added to yet, so when you run ‘pytest’ now, the result should look like this:

A couple of other small preliminaries to get testing set up:

  • Set WTF_CSRF_ENABLED configuration variable equal to False in the testing configuration found in This is required for tests to run properly. Keep in mind that you normally want to have this set equal to True when you’re running your application in production, since this protects your forms against Cross Site Request Forgery (CSRF) attacks.
  • Add in your /tests directory.

Adding Tests

The great thing about pytest is that it allows you to write tests in a very concise, pythonic way using assert statements. Suppose you have the following file which defines a function that takes the square of a number. You want to test this. Adding a couple of tests is as easy as this:

def squared(x):
return x**2
def test_squared():
assert squared(3) == 9
assert squared(3) == 10

Then, you run your tests by typing `pytest` in the command line when you’re in the app’s main directory. This returns the following result (one test pass, one test fail):

Another great feature of pytest is the ability to produce “fixtures”, which provides tests with pre-initialized objects you require to run your tests. In our case, a great use of fixtures is initializing the test database or the test application instance.

Pytest fixtures have some advantages over the usual setup and teardown functions used in other testing frameworks. One is the ability to run fixtures using different “scopes”, allowing you to do the setup / teardown operation at different times when you run your test to maximize test efficiency. The options for scope here include function, class, module, and session. So, for example, a fixture in the function scope means that the fixture object is invoked once for each test function you define in pytest.

So, say all your tests need to be able to access the Flask test client application instance. Using the module fixture, the application instance is only created once at the very beginning when you run your test, rather than being created and destroyed once for every test function.

For our small app, we’ll create 3 fixtures:

  • A database record in our Name table
  • The Flask application instance
  • The test database

The following code creates a fixture for one database record in our Name table which will make this object available to all our tests.

import pytest
from app.models import Name
def new_name():
name = Name(name='Mark')
return name

As you can see, you register fixtures using the pytest.fixtures decorator. This function returns the fixture object that you want your tests to access. You access this fixture object in your tests by supplying them as an argument to the test function. The code below shows a test we can add to that accesses the new_name fixture and ensures it has the value we expect.

from .conftest import new_name
def test_new_name(new_name):
assert == 'Mark'

We can take advantage of the work we did in Part 2 of the series using our application factory pattern to easily create a fixture for our application instance with our testing configuration options. The code below does this by creating an application instance with testing configuration and returning its test client (the test client provides a simple interface to the application where we can trigger requests application and track cookies).

import pytest
from app import create_app
def test_client():
app = create_app('testing')
testing_client = app.test_client()
ctx = app.app_context()
yield testing_client

Note that the part after the yield statement represents the “teardown” part of the test: this is where the application instance is removed when the tests are done running.

We also need to create a similar fixture for our database. The following code creates the database and initializes the data with create_all(), adds one Name “Mark” and then cleans up the database after the tests are complete.

import pytest
from app.models import Name
from app import db
def create_database():
name = Name(name='Mark')
yield db

Right now, when out user enters information into the app’s simple web form, the data is simply added to the database and nothing changes from the user’s perspective. To add some more interesting system tests, let’s add a little bit of additional functionality to our application: it will now print out a list of all the existing names in the database on the main page.

Finally, we add a few more tests to use all of our new fixtures, including tests to make sure our application instance exists and ensure the HTML output produced contains data we expect. 

from .conftest import new_name, test_client, create_database
def test_new_name(new_name):
assert == 'Mark'
def test_test_client(test_client):
assert test_client is not None
def test_main_page(test_client, create_database):
response = test_client.get('/')
assert response.status_code == 200
assert b'Hello World!' in
assert b'Mark' in

(Note that “Mark” appears on the main page since this record was added to the database as part of the fixture.)

Now, in your main app directory, you can run your tests by simply typing “pytest” in the command line. The result for our code here should look like this:

The three dots ‘…’ indicate that there were three tests and they each passed (if a test failed, one of these dots appear as an F).

Aside: during the creation of this post, I was puzzled to see that my code was running in the flask production environment, despite my configuration and environment variables clearly indicating that I was in development. Turns out there is another environment variable that needs to be set called FLASK_ENV, which defaults to “production”, turning off debug mode in Flask and throwing a warning when you run your application on the Flask development server.  To fix this, run `export FLASK_ENV=“development”` on the command line. Note that this must be set as an environment variable: adding it in doesn’t work. I’ve updated this information in Part 2.  For further information in the Flask documentation, see here.


Digging into Data Science Tools: Docker

Docker is a tool for creating and managing “containers” which are like little virtual machines where you can run your code. A Docker container is like a little Linux OS, preinstalled with everything you need to run your web app, machine learning model, script, or any other code you write.

Docker containers are like a really lightweight version of virtual machines. They use way less computer resources than a virtual machine, and can spin up in seconds rather than minutes. (The reason for this performance improvement is Docker containers share the kernel of the host machine, whereas virtual machines run a separate OS with a separate kernel for every virtual machine.)

Aly Sivji provides a great comparison of Docker containers to shipping containers. Shipping containers improved efficiency of logistics by standardizing the design: they all operate the same way and we have standardized infrastructure for dealing with them, and as a result you can ship them regardless of transportation type (truck, train, or boat) and logistics company (all are aware of shipping containers and mold to their standards). In a similar way, Docker provides a standardized software container which you can pass into different environments and be confident they’ll run as you expect.  

Brief Overview of How Docker Works

To give you a really high-level overview of how Docker works, first let’s define three big Docker-related terms – “Dockerfile”, “Image”, and “Container”:

  • Dockerfile: A text file you write to build the Docker “image” that you need (see definition of image below). You can think of the Dockerfile like a wrapper around the Linux command line: the commands that you would use to set up a Linux system on the command line have equivalents which you can place in a docker file. “Building” the Dockerfile produces an image that represents a Linux machine that’s in the exact state that you need. You can learn all about the ins-and-outs of the syntax and commands at the Dockerfile reference page. To get an idea of what Dockerfiles look like, here is a Dockerfile you would use to create an image that has the Ubuntu 15.04 Linux distribution, copy all the files from your application to ./app in the image, run the make command on /app within your image’s Linux command line, and then finally run the python file defined in /app/
FROM ubuntu:15.04
COPY . /app
RUN make /app
CMD python /app/
  • Image: A “snapshot” of the environment that you want the containers to run. The images include all you need to run your code, such as code dependencies (e.g. python venv or conda environment) and system dependencies (e.g. server, database). You “build” images from Dockerfiles which define everything the image should include. You then use these images to create containers.
  • Container: An “instance” of the image, similar to how objects are instances of classes in object oriented programming. You create (or “run” using Docker language) containers from images. You can think of containers as a running the “virtual machine” defined by your image.

To sum up these three main concepts: you write a Dockerfile to “build” the image that you need, which represents the snapshot of your system at a point in time. From this image, you can then “run” one or more containers with that image.

Here are a few other useful terms to know:

  • Volume: “Shared folders” that lets a docker container see the folder on your host machine (very useful for development, so your container is automatically updated with your code changes). Volumes also allow one docker container to see data in another container. Volumes can be “persistent” (the volume continues to exist after the container is stopped) or “ephemeral” (the volume disappears as soon as the container is stopped).
  • Container Orchestration: When you first start using Docker, you’ll probably just spin up one container at a time. However, you’ll soon find that you want to have multiple containers, each running using a different image with different configurations. For example, a common use of Docker is deployment of applications as “microservices”, where each Docker container represents an individual microservice that interacts with your other microservices to deliver your application. Since it can get very unwieldy to manage multiple containers manually, there are “container orchestration” tools that automate tasks such as starting up all your containers, automatically restarting failing containers, connecting containers together so they can see each other, and distributing containers across multiple computers. Examples of tools in this space include docker-compose and Kubernetes.
  • Docker Daemon / Docker Client: The Docker Daemon must be running on the machine where you want to run containers (could be on your local or remote machine). The Docker Client is front-end command line interface to interact with Docker, connect to the Docker Daemon, and tell it what to do. It’s through the Docker client where you run commands to build images from Dockerfiles, create containers from images, and do other Docker-related tasks.

Why is Docker useful to Data Scientists?

You might be thinking “Oh god, another tool for me to learn on top of the millions of other things I have to keep on top of? Is it worth my time to learn it? Will this technology even exist in a couple years?

I think the answer is, yes, this is definitely a worthwhile tool for you to add to your data science toolbox.

To help illustrate, here is a list of reasons for using Docker as a data scientist, many of which are discussed in Michael D’agostino’s “Docker for Data Scientists” talk as well as this Lynda course from Arthur Ulfeldt:

  • Creating 100% Reproducible Data Analysis: Reproducibility is increasingly recognized as critical for both methodological and legal reasons. When you’re doing analysis, you want others to be able to verify your work. Jupyter notebooks and Python virtual environments are a big help, but you’re out of luck if you have critical system dependencies. Docker ensures you’re running your code in exactly the same way every time, with the same OS and system libraries.
  • Documentation: As mentioned above, the basis for building docker containers is a “Dockerfile”, which is a line by line description of all the stuff that needs to exist in your image / container. Reading this file gives you (and anyone else that needs to deploy your code) a great understanding about what exactly is running on the container.
  • Isolation: Using Docker helps ensure that your tools don’t conflict with one another. By running them in separate containers, you’ll know that you can run Python 2, Python 3, and R and these pieces of software will not interfere with each other.
  • Gain DevOps powers: in the words of Michaelangelo D’Agostino, “Docker Democratizes DevOps”, since it opens up opportunities to people that used to only available to systems / DevOps experts:
    • Docker allows you to more easily “sidestep” DevOps / system administration if you aren’t interested, since someone can create a container for you and all you have to do it run it. Similarly, if you like working with Docker,  you can create a container less technically savvy coworkers that lets them run things easily in the environment they need.
    • Docker provides the ability to build docker containers starting from existing containers. You can find many of these on DockerHub, which holds thousands of pre-built Dockerfiles and images. So if you’re running a well-known application (or even obscure applications), there is often a Dockerfile already available that can give you a tremendous running start to deploy your project. This includes “official” Docker repositories for many tools, such as ubuntu, postgres, nginx, wordpress, python, and much more.
    • Using Docker helps you work with your IT / DevOps colleagues, since you can do your Data Science work in a container, and simply pass it over to DevOps as a black box that they can run without having to know everything about your model.

Here are a few examples of applications relevant to data science where you might try out with Docker:

  • Create an ultra-portable, custom development workflow: Build a personal development environment in a Dockerfile, so you can access your workflow immediately on any machine with Docker installed. Simply load up the image wherever you are, on whatever machine you’re on, and your entire work environment is there: everything you need to do your job, and how you want to do your job.
  • Create development, testing, staging, and production environments: Rest assured that your code will run as you expect and become able to create staging environments identical to production so you know when you push to production, you’re going to be OK.
  • Reproduce your Jupyter notebook on any machine: Create a container that runs everything you need for your Jupyter Notebook data analysis, so you can pass it along to other researchers / colleagues and know that it will run on their machine. As great as Jupyter Notebooks are for doing analysis, they tend to suffer from the “it works on my machine” issue, and Docker can solve this issue.

For more inspiration, check out Civis Analytics Michaelangelo D’Agostino describe the Docker containers they use (start at the 18:08 mark). This includes containers specialized for survey processing, R shiny apps and other dashboards, Bayesian time series modeling and poll aggregation, as well as general purpose R/Python packages that have all the common packages needed for staff.

Further Resources

If you’re serious about starting to use Docker, I highly recommend the Lynda Course Learning Docker by Arthur Ulfeldt as a starting point. It’s well-explained and concise (only about 3 hours of video in total). I created a set of Anki flashcards from this course you can access here. I also recommend the book Docker Deep Dive by Nigel Poulton. I also created Anki flashcards from this book that you can access here.

Here are a few other useful resources you might want to check out: