Deploying a Python Flask Example Application Using Heroku

Initializing the Project

The project initialization consists of creating a directory for your application, setting up a Python virtual environment where dependencies will be installed, and initializing the Git repository. You don’t have to use a virtual environment or Git for local development, but they are very convenient and will make development and deployment to Heroku simpler.

Start by creating a new directory for the Python Flask example app. You can do it by running these commands:

$ mkdir realpython-example-app
$ cd realpython-example-app

The above commands create a realpython-example-app/ folder and change the current working directory to it.

Next, you must create a Python virtual environment. Using a virtual environment allows you to manage your project’s dependencies without messing with system-level files shared by all applications. Run the following commands to create and activate a virtual environment for your application. Remember to change the directory to realpython-example-app/ if you haven’t already:

$ python3 -m venv venv
$ source venv/bin/activate

These commands create a virtual environment named venv and activate it, so packages will be loaded and installed from this environment instead of using the system-level packages.

Installing Dependencies

The first step is to install the Flask Python package. You can run the following command to install Flask using pip:

$ python3 -m pip install Flask==1.1.2

The above command installs Flask version 1.1.2. This is the version you’ll use throughout this tutorial. Next, you need to create a requirements.txt file listing the project’s dependencies. You can use the python3 -m pip freeze command for this task:

$ python3 -m pip freeze > requirements.txt

You’ll use requirements.txt when deploying the project to tell Heroku which packages must be installed to run your application code. To learn more about how to install Flask in a virtual environment and other pip options, check out the Flask installation documentation and What Is Pip? A Guide for New Pythonistas.

Now, the application directory should look like this:

realpython-example-app/
│
├── venv/
│
└── requirements.txt

In the following sections, you’ll add more files to implement the application logic, set up Git, and then deploy it to Heroku.

Writing the Application Code

In this section, you’ll create a small Flask application with a single route, index, that returns the text Hello World! when requested. To create a Flask application, you have to create a Flask object that represents your app and then associate views to routes. Flask takes care of dispatching incoming requests to the correct view based on the request URL and the routes you’ve defined.

You can click the link below to get the full source code for this tutorial’s application:

For small applications, like the one you’re working with in this tutorial, you can write all the code in a single file, organizing your project as follows:

realpython-example-app/
│
├── venv/
│
├── app.py
└── requirements.txt

app.py contains the application’s code, where you create the app and its views. The following code block shows the application’s full source code:

from flask import Flask

app = Flask(__name__)

@app.route("/")
def index():
    return "Hello World!"

After you import flask, the code creates the object app, which belongs to the Flask class. The view function index() is linked to the main route using the app.route() decorator. When the main route is requested, Flask will serve the request by calling index() and using its return value as the response.

Running the Python Flask Example Locally

There are different ways in which you can run the application that you created in the previous section. One of the most straightforward ways to launch a Flask app for local development is using the flask run command from a terminal:

$ flask run

By default, Flask will run the application you defined in app.py on port 5000. While the application is running, go to http://localhost:5000 using your web browser. You’ll see a web page containing the message Hello World!

During development, you normally want to reload your application automatically whenever you make a change to it. You can do this by passing an environment variable, FLASK_ENV=development, to flask run:

$ FLASK_ENV=development flask run

When you set FLASK_ENV=development, Flask will monitor changes to app files and reload the server when there’s a change. This way, you don’t need to manually stop and restart the app server after each modification.

Tracking Changes Using Git

In this tutorial, you’re going to track the changes to your project’s files using Git, a very popular version control system (VCS). So, as a first step, you should create a Git repository for your project. You can achieve this by executing the following command in your project’s directory:

$ git init

The above command initializes the repository that will be used to track the project’s files. The repository metadata is stored in a hidden directory named .git/.

Note that there are some folders that you shouldn’t include in the Git repository, like venv/ and __pycache__/. You can tell Git to ignore them by creating a file named .gitignore. Use the following commands to create this file:

$ echo venv > .gitignore
$ echo __pycache__ >> .gitignore
$ git add .gitignore app.py requirements.txt
$ git commit -m "Initialize Git repository"

After running the above commands, Git will track changes to your application’s files, but it will ignore the venv/ and __pycache__/ folders. Now the project directory, realpython-example-app/, should look like this:

realpython-example-app/
│
├── .git/
│
├── venv/
│
├── .gitignore
├── app.py
└── requirements.txt

You’re now ready to deploy your app using Heroku. Check out Introduction to Git and GitHub for Python Developers to learn more about Git and how you can host your repository in GitHub.

Heroku Account Setup

Your first step is to create a Heroku account. If you don’t have one already, you can use the Free and Hobby plan. It allows you to deploy noncommercial applications, personal projects, and experiments without spending money.

If you go to the Heroku sign-up page, you’ll see the following fields on the sign-up form:

You’ll be able to start using Heroku after completing the required information and confirming your email address.

Heroku Command-Line Interface (CLI)

The Heroku command-line interface (CLI) is a tool that allows you to create and manage Heroku applications from the terminal. It’s the quickest and the most convenient way to deploy your application. You can check the developer’s documentation for installation instructions for your operating system. On most Linux distributions, you can install the Heroku CLI by running the following command:

$ curl https://cli-assets.heroku.com/install.sh | sh

The previous command downloads the Heroku CLI installer and executes it. Next, you have to log in by running the following command:

$ heroku login

This opens a website with a button to complete the login process. Click Log In to complete the authentication process and start using the Heroku CLI:

After logging in, you’re ready to start using the Heroku CLI to manage your applications and workflows.

Application Deployment to Heroku

In this section, you’ll learn how to use the Heroku CLI and Git to deploy your web application. The first step is to create a file named Procfile in the project’s root directory. This file tells Heroku how to run the app. You can create it by running the following command:

$ echo "web: gunicorn app:app" > Procfile

Note that this filename must start with a capital letter. This file tells Heroku to serve your application using Gunicorn, a Python Web Server Gateway Interface (WSGI) HTTP server compatible with various web frameworks, including Flask. Make sure to install Gunicorn and update the requirements.txt file using pip:

$ python3 -m pip install gunicorn==20.0.4
$ python3 -m pip freeze > requirements.txt

The previous commands install Gunicorn and update requirements.txt to contain the list of all dependencies. Since you added and changed files, you need to commit them to Git. You can do this by executing the following two commands:

$ git add Procfile requirements.txt
$ git commit -m "Add Heroku deployment files"

When you execute the above commands, you’ll commit the latest versions of Procfile and requirements.txt to the Git repository. Now you’re ready to deploy the application to Heroku. Start by creating a Heroku application using the Heroku CLI.

You can create the application in Heroku by running the following command:

$ heroku create realpython-example-app

Running the above command initializes the Heroku application, creating a Git remote named heroku. Next, you can push the Git repository to this remote to trigger the building and deployment process:

$ git push heroku master

After pushing the master branch to the heroku remote, you’ll see that the output displays information about the building and deployment process:

 1Enumerating objects: 6, done.
 2Counting objects: 100% (6/6), done.
 3Delta compression using up to 8 threads
 4Compressing objects: 100% (4/4), done.
 5Writing objects: 100% (6/6), 558 bytes | 558.00 KiB/s, done.
 6Total 6 (delta 0), reused 0 (delta 0)
 7remote: Compressing source files... done.
 8remote: Building source:
 9remote:
10remote: -----> Building on the Heroku-18 stack
11remote: -----> Python app detected
12remote: -----> Installing python-3.6.12
13remote: -----> Installing pip 20.1.1, setuptools 47.1.1 and wheel 0.34.2
14remote: -----> Installing SQLite3
15remote: -----> Installing requirements with pip
16remote:        Collecting click==7.1.2
17remote:          Downloading click-7.1.2-py2.py3-none-any.whl (82 kB)
18remote:        Collecting Flask==1.1.2
19remote:          Downloading Flask-1.1.2-py2.py3-none-any.whl (94 kB)
20remote:        Collecting itsdangerous==1.1.0
21remote:          Downloading itsdangerous-1.1.0-py2.py3-none-any.whl (16 kB)
22remote:        Collecting Jinja2==2.11.2
23remote:          Downloading Jinja2-2.11.2-py2.py3-none-any.whl (125 kB)
24remote:        Collecting MarkupSafe==1.1.1
25remote:          Downloading MarkupSafe-1.1.1-cp36-cp36m-manylinux1_x86_64.whl
26remote:        Collecting Werkzeug==1.0.1
27remote:          Downloading Werkzeug-1.0.1-py2.py3-none-any.whl (298 kB)
28remote:        Installing collected packages: click, Werkzeug, itsdangerous,
29MarkupSafe, Jinja2, Flask
30remote:        Successfully installed Flask-1.1.2 Jinja2-2.11.2 MarkupSafe-1.1.1
31Werkzeug-1.0.1 click-7.1.2 itsdangerous-1.1.0
32remote: -----> Discovering process types
33remote:        Procfile declares types -> (none)
34remote:
35remote: -----> Compressing...
36remote:        Done: 45.1M
37remote: -----> Launching...
38remote:        Released v1
39remote:        https://realpython-example-app.herokuapp.com/ deployed to Heroku
40remote:
41remote: Verifying deploy... done.
42To https://git.heroku.com/realpython-example-app.git
43 * [new branch]      master -> master

Congratulations, the app is now online! The output shows the building process, including the installation of dependencies and the deployment. On line 39, you’ll find the URL for your application. In this case, it’s https://realpython-example-app.herokuapp.com/. You can also use the following Heroku CLI command to open your app’s URL:

$ heroku open

The above command will open your application using your default web browser.

Now let’s make a small change to the app and see how you can redeploy it. Edit app.py and modify the string returned by index() as shown in the next code block:

 1from flask import Flask
 2
 3app = Flask(__name__)
 4
 5@app.route("/")
 6def index():
 7    return "Hello this is the new version!"

As you see on line 7, "Hello World!" was replaced by "Hello this is the new version!" You can deploy this version to Heroku by committing and pushing the changes to the heroku remote:

$ git add app.py
$ git commit -m "Change the welcome message"
$ git push heroku master

With these commands, you commit the changes to the local Git repository and push them to the heroku remote. This triggers the building and deployment process again. You can repeat these steps whenever you need to deploy a new version of your application. You’ll notice that subsequent deployments usually take less time because the requirements are already installed.

In this section, you learned how to create and deploy your application on Heroku using Git and the Heroku CLI. For more details about using the Heroku CLI to deploy Python applications, check out Getting Started on Heroku with Python.

Understanding the Deployment Workflow

As you saw above, when you use this workflow, you’ll run the application in three separate environments:

1. Development is the local environment.
2. Staging is the preproduction environment used for previews and testing.
3. Production is the live site accessed by final users.

In previous sections, you saw how to run the application on your local environment and in the production environment on Heroku. Adding a staging environment can greatly benefit the development process. The main purpose of this environment is to integrate changes from all new branches and to run the integration tests against the build, which will become the next release.

Next, you’ll see how to create the staging environment in Heroku and how to create a pipeline to promote versions from staging to production. The following diagram shows this workflow:

The above image shows the three environments, the activities that happen in each of them, and the deployment and promotion steps.

Implementing the Deployment Workflow in Heroku

Implementing the workflow in Heroku consists of two steps:

1. Creating separate applications for staging and production
2. Making both applications part of the same pipeline

A Heroku pipeline is a group of applications tied together by a workflow. Each one of these applications is an environment in the development workflow, like staging or production. Using pipelines guarantees that, after promotion, production will run the exact same code that you reviewed in staging.

In this tutorial, the previously created Heroku application realpython-example-app is used as the production environment. You should create a new Heroku app for the staging environment using these commands:

$ heroku create realpython-example-app-staging --remote staging
$ git push staging master

Running these commands creates a new Heroku app named realpython-example-app-staging and deploys the application to it using Git. You can then access the staging app at https://realpython-example-app-staging.herokuapp.com/. Note that a Git remote named staging is associated with this application.

Now that you have Heroku applications for production and staging, you’re ready to create a Heroku pipeline that links them together. You can use the Heroku CLI to create the pipeline:

$ heroku pipelines:create --app realpython-example-app \
    --stage production \
    realpython-example-app
Creating realpython-example-app pipeline... done
Adding ⬢ realpython-example-app to realpython-example-app pipeline as production
... done

The command above creates a pipeline named realpython-example-app and adds the app named realpython-example-app as the production environment. Next, run the following command to create a Git remote that points to this app, naming it prod:

$ heroku git:remote --app realpython-example-app --remote prod

From now on, you can refer to the production deployment as prod.

Next, add the staging application to the same pipeline by running the following command:

$ heroku pipelines:add realpython-example-app \
  --app realpython-example-app-staging \
  --stage staging
Adding ⬢ realpython-example-app-staging to realpython-example-app pipeline as
staging... done

This command adds the app realpython-example-app-staging to the same pipeline and specifies that this app must be used for the staging stage. This means that your pipeline now consists of two apps:

1. realpython-example-app
2. realpython-example-app-staging

The first one is used as the production environment, and the second one is used as the staging environment.

Deploying and Promoting to Staging and Production

Now that you have your applications and pipeline configured, you can use it to deploy your application to staging, review it there, and then promote it to production.

Suppose for example that you want to change the message returned by the index() view again. In that case, you have to edit app.py and change the string returned by index(). The following code block shows the new version:

 1from flask import Flask
 2
 3app = Flask(__name__)
 4
 5@app.route("/")
 6def index():
 7    return "This is yet another version!"

As you see on line 7, index() returns "This is yet another version!" You can deploy this new version to your staging environment by running the following commands:

$ git add app.py
$ git commit -m "Another change to the welcome message"
$ git push staging master

These commands commit app.py and push the changes to the staging remote, triggering the building and deployment process for this environment. You should see the new version deployed at https://realpython-example-app-staging.herokuapp.com/. Note that the production environment is still using the previous version.

When you’re happy with the changes, you can promote the new version to production using the Heroku CLI:

$ heroku pipelines:promote --remote staging

The above command deploys to production the exact same version that is currently running in staging. As you’ll notice, in this case, there’s no build step since the same build from staging is used and deployed to production. You can verify at https://realpython-example-app.herokuapp.com/ that the application was promoted and that it’s running the latest version.

In this section, you learned about the deployment workflow and implemented it in Heroku. To learn more about working with pipelines and more advanced workflows, check out the Heroku pipelines documentation.