CoCalc Blog

Dash with CoCalc

Hal Snyder • • cocalc and python

Create interactive data visualizations for collaborators in your CoCalc projects using Dash.

Dash is an open-source framework to create web applications with Python. With CoCalc’s HTTPWebserver capability, you can run a Dash application from inside a CoCalc project.


Dash application running in a CoCalc project

We’ll step through a working program based on the first example in the Dash Tutorial, with a few additions for configuring the HTTP server. Code for the completed CoCalc example is viewable on the share server.

Start by installing and updating Python3 packages in your project from a .term file:

pip3 install --user dash
pip3 install --user dash_core_components
pip3 install --user dash_html_components
pip3 install --user --upgrade plotly

Copy the first example python file, “Hello Dash”, from the tutorial Python excerpt app.py into a .py file in your project. The code is short enough that you can use copy and paste.

Insert the following three code snippets just above the test for __name__ == '__main__'.

Set the HTTP port number for your Dash web app. You can probably use the default port number of 8050, but you will need to specify the port number in the prefix regardless. Our example uses 9990.

port = 9990

Get your project ID from the environment variable COCALC_PROJECT_ID. That is the specially-formatted hex code you see in the browser URL when logged into your project, and displayed the Project Settings tab under Project Control.

import os
cocalc_project_id = os.environ['COCALC_PROJECT_ID']

Configure requests_pathname_prefix as indicated in the HTTPWebserver wiki page:

pfx = "/{}/server/{}/".format(cocalc_project_id, port)
app.config.requests_pathname_prefix = pfx

In the final line, display the URL for your server and override default port number and host in the call to app.run_server():

if __name__ == '__main__':
    print("browse to https://cocalc.com{}".format(pfx))
    app.run_server(debug=True, port=port, host='0.0.0.0')

Back in your .term session, run the program. For example, if the file you just edited is app.py, do this:

python3 app.py

Copy the URL that is displayed by the program. Open a new browser tab and paste in the URL. You will see the “Hello Dash” demo.

Note: Because of the way CoCalc security works, only collaborators on the given project can access the webserver. Our proxy server will reject all other requests.

More examples are available in the Dash User Guide and Introducing Dash.

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