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Flask

Note: These instructions are primarily intended for individual user accounts. If you are using a group account, you may wish to consider apphosting instead.

Flask is a popular microframework for Python web development. Using it on the OCF servers requires only just a little extra configuration.

Setting up a Flask project

  1. Make a new directory for your flask app in your home directory. You can name this whatever you want, but we'll assume you named the directory app:

    user@tsunami:~$ mkdir app
    user@tsunami:~$ cd app
    
  2. Make a virtualenv. This will allow you to run pip install to install any python packages that you need that the OCF does not already have. The benefit of having a virtualenv is that you can decide the dependencies you want, without being tied to the OCF defaults:

    user@tsunami:~/app$ virtualenv -p python3 venv
    user@tsunami:~/app$ venv/bin/pip install flask flup6
    

    Feel free to install any other packages you need for your flask app at this step too (or later on, that's cool too). If you have an existing flask app with a requirements.txt file, then run venv/bin/pip install -r requirements.txt to install dependencies from that.

    For full details on how to use pip, see the pip documentation.

  3. Create a new python file (or directory) for your application logic. In this case, we'll create a new directory named myapp and make a new file within it named app.py:

    from flask import Flask
    app = Flask(__name__)
    
    @app.route('/')
    def hello():
        return 'Hello World!'
    
    if __name__ == '__main__':
        app.run()
    
  4. Create a new directory in your ~/public_html/ directory for your flask application. You can also just put these files directly in ~/public_html if you want your site available directly under https://www.ocf.berkeley.edu/~user and you don't have any other sites:

    user@tsunami:~$ mkdir ~/public_html/flask
    

    For example, this path above will make your site available at https://www.ocf.berkeley.edu/~user/flask.

  5. Create ~/public_html/flask/.htaccess with these contents:

    RewriteEngine On
    RewriteBase /
    RewriteCond %{REQUEST_FILENAME} !-f
    # Change "user" and "flask" to your username and whatever directory
    # name you made in public_html, respectively
    RewriteRule ^(.*)$ /~user/flask/run.fcgi/$1 [QSA,L]
    
  6. Create ~/public_html/flask/run.fcgi with these contents:

    #!/home/u/us/user/app/venv/bin/python
    import os
    import sys
    
    sys.path.insert(0, os.path.expanduser('~/app'))
    from flup.server.fcgi import WSGIServer
    from myapp.app import app
    
    if __name__ == '__main__':
        WSGIServer(app).run()
    

    Make sure to replace the first line of run.fcgi file with the actual path to your project's virtualenv Python. You can find it by running the command readlink -f ~/app/venv/bin/python.

    Make sure to also replace ~/app (if you have a different path in your home directory) and from myapp.app with the name of your Flask application that you selected before (we used myapp and app.py but you can use whatever you want).

  7. Make the run.fcgi file you just created executable with:

    user@tsunami:~$ chmod +x ~/public_html/flask/run.fcgi
    
  8. Once your app has started running, changes you make to the Python code or templates won't take effect for a few hours. To apply changes immediately, the webserver needs to see that the run.fcgi file has changed. You can can change the modification time of the run.fcgi file to trigger a restart with the command:

    user@tsunami:~$ touch ~/public_html/flask/run.fcgi
    

Debugging

If you see an error page when trying to load your app, you may find the webserver's logs useful. You can access them in the following locations:

  • error log: /opt/httpd/error.log (most useful)
  • suexec log: /opt/httpd/suexec.log (only useful in rare cases)