So - you know some Python, HTML and CSS, and you've ventured into Flask for the first time. You've put together your first app, and you are now in a position to deploy it. Unfortunately, however, this can't be done at the click of a button (initially at least!). The deployment of your first Flask app is certainly harder than putting together the app itself. The deployment is a skill in itself, and you should approach it in exactly the same way as you approach learning to code - start with a very, very simple example, and build up your knowledge through lots of practice. Eventually, the deployment will become second nature.

There are several popular platforms out there for deploying Flask apps. The top 3 are probably AWS, Google App Engine and Heroku. This article will talk you through the deployment of the most basic Flask app (without any CSS or static files eg images) to Heroku, using a Windows environment. Of course, I am 99.9% confident that your Flask app contains CSS files - however, it is best that you start with the simplest Flask deployment possible and then progress from there.


>>> import sys
>>> sys.version
'2.7.9 | 64-bit | (default, Jul  1 2015, 03:41:50) [MSC v.1500 64 bit (AMD64)]'
Documents> pip install virtualenv

The majority of problems encountered when deploying your first Flask app to Heroku will be caused by problems with git, pip or virtualenv. If you do encounter problems in the steps that follow, it is thoroughly recommended to go back and practice working with git, pip and virtualenv until you really understand them, and are confident in using them.

1. Getting Started

Navigate to your Documents folder in Powershell, and run

Documents> mkdir first_flask_app
Documents> cd first_flask_app
Documents/first_flask_app> new-item

We will use the Flask documentation hello world application. Put the following content into your file:

from flask import Flask
app = Flask(__name__)

def hello_world():
    return 'Hello World!'

if __name__ == '__main__':

To run the app locally, use the following command:

Documents/first_flask_app> python
* Running on (Press CTRL+C to quit)

You can then navigate to in your browser to see the 'Hello World' output.

You have now verified that your basic Flask app is running locally, and are ready to deploy to Heroku.

2. Setting up your Virtual Environment

Next, you need to set up a virtual environment in your project folder. This virtual environment will be used to tell Heroku which Python packages your Flask app needs to use. For example, if your Flask app uses numpy to do some numerical processing on the backend, you will need to include numpy in your virtual environment. Navigate to the root of your project folder and run:

Documents/first_flask_app> virtualenv venv
Documents/first_flask_app> cd venv/Scripts

To activate your virtual environment, run:

Documents/first_flask_app/venv/Scripts> .\activate

(venv) should now appear at the beginning of your path in Powershell. You can deactivate the virtual environment in Powershell at any time by running deactivate.

Now that your virtual environment is activated, you are in a position to install the Python packages that Heroku requires for Flask. The packages required are as follows:

You can install all of these packages with:

Documents/first_flask_app> pip install flask gunicorn jinja2

However, if you encounter error messages while installing these packages then you might have to install each one individually (again, with pip), and debug any errors which occur.

3. Create a Procfile (Process File)

Each Flask app deployed to Heroku requires a Procfile. Navigate to the root of the project and run:

Documents/first_flask_app> new-item Procfile

Do not make the Procfile a text file. Open the Procfile in an editor (eg Notepad) and paste the following into your Procfile:

web: gunicorn app:app

4. Create a Requirements File

Heroku recognises the packages used in your Flask app by looking inside the requirements.txt file. You create your requirements file using pip and virtualenv. Running pip freeze requirements will display the packages that you have installed in your virtual environment.

Documents/first_flask_app> pip freeze requirements

You will note that additional packages (itsdangerous, MarkupSafe and Werkzeug) have been installed. This is because running pip install flask automatically installs itsdangerous andWerkzeug, and running pip install jinja2 automatically installs MarkupSafe.

To save these requirements to your requirements.txt file, run pip freeze > requirements.txt.

Documents/first_flask_app> pip freeze > requirements.txt

Your requirements.txt file will look like this:


Key Step: in Windows, your requirements.txt file will be in Unicode format by default. What the official Heroku documentation doesn't mention is that the requirements.txt file must be saved in ANSI format for the Flask app to be deployed to successfully. Open your requirements.txt file in an editor (eg Notepad), click 'Save As' and change the encoding to ANSI. The following image shows the file after I have updated the encoding:

save heroku requirements file as ANSI

5. Save your Flask app using Git

First, create a .gitignore file:

Documents/first_flask_app> new-item .gitignore

Open the .gitignore file and write venv. This will make git ignore the venv directory - we can do this because Heroku doesn't need to see the venv directory - it can get all the information it needs about Python packages from the requirements.txt file. Next, initialise your git repository and make a first commit:

Documents/first_flask_app> git init
Documents/first_flask_app> git add --all
Documents/first_flask_app> git commit -m "first commit with app ready for deployment"

6. Deploy your app to Heroku

Login to the Heroku toolbelt using:

Documents/first_flask_app> heroku login

Enter your Heroku email address, username and password in Powershell if prompted. Then run

Documents/first_flask_app> heroku create
Creating polar-tor-1665... done, stack is cedar-14 |
Git remote heroku added
heroku-cli: Updating... done.

This creates a Heroku app at and adds a Git remote called heroku so that when you push your repository to the heroku remote, it will update the app URL. I can view the heroku remote in the command line with:

Documents/first_flask_app> git remote -v
heroku (fetch)
heroku (push)

To deploy the app to Heroku, run

Documents/first_flask_app> git push heroku master

Depending on your internet connection, your app may take anywhere betwen 15 seconds and 10 minutes (or more!) to upload. The following messages appear in Powershell:

Counting objects: 1523, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (1477/1477), done.
Writing objects: 100% (1523/1523), 5.79 MiB | 67.00 KiB/s, done.
Total 1523 (delta 241), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote: -----> Python app detected
remote: -----> Installing runtime (python-2.7.11)
remote: -----> Installing dependencies with pip
remote:        Collecting Flask==0.10.1 (from -r requirements.txt (line 1))
remote:        Downloading Flask-0.10.1.tar.gz (544kB)
remote:        Collecting gunicorn==19.4.5 (from -r requirements.txt (line 2))
remote:        Downloading gunicorn-19.4.5-py2.py3-none-any.whl (112kB)
remote:        Collecting itsdangerous==0.24 (from -r requirements.txt (line 3))
remote:        Downloading itsdangerous-0.24.tar.gz (46kB)
remote:        Collecting Jinja2==2.8 (from -r requirements.txt (line 4))
remote:        Downloading Jinja2-2.8-py2.py3-none-any.whl (263kB)
remote:        Collecting MarkupSafe==0.23 (from -r requirements.txt (line 5))
remote:        Downloading MarkupSafe-0.23.tar.gz
remote:        Collecting Werkzeug==0.11.3 (from -r requirements.txt (line 6))
remote:        Downloading Werkzeug-0.11.3-py2.py3-none-any.whl (305kB)
remote:        Installing collected packages: Werkzeug, MarkupSafe, Jinja2, itsdangerous, Flask, gunicorn
remote:        Running install for MarkupSafe
remote:        Running install for itsdangerous
remote:        Running install for Flask
remote:        Successfully installed Flask-0.10.1 Jinja2-2.8 MarkupSafe-0.23 Werkzeug-0.11.3 gunicorn-19.4.5 itsdangero
remote: -----> Discovering process types
remote:        Procfile declares types -> web
remote: -----> Compressing... done, 42.8MB
remote: -----> Launching...
remote:        Released v3
remote: deployed to Heroku
remote: Verifying deploy... done.
 * [new branch]      master -> master

Of course, your app will not be deploying to polar-tor-1665 like mine - it will be using another app name. You can now visit your app at the designated URL shown in Powershell (mine is

flask heroku app live

7. Key Tips

If you are having problems with any of the above steps, then go back and read, re-read and re-read this article. Make sure you are following each step exactly as I show you - you have to be precise. If you are new to any of git, pip or virtualenv then go away and practice them until you understand them more comprehensively and are confident when using them. This will make the process of deploying Flask apps to Heroku quicker and easier.

8. Code

The code for this project (minus the virtual environment) is available at