App callback without an output

Dash Python
7

In cases like these, you can “store” the output in a hidden DIV as JSON and then use that result in the next callbacks. See Add reactive expressions / blocks · Issue #49 · plotly/dash · GitHub for more details.

The main thing is that the Dash app backend’s state is read-only. The real “state” of the app is inside the front-end in the user’s web browser. Dash apps can’t modify global variables in the Python context because those modifications

  • wouldn’t scale across multiple python processes (each with their own memory). when dash apps are deployed, they run on multiple processes.
  • would need to be stored separately for each user, increasing the memory requirements of the backend considerably. instead, we store the state in the front-end in the user’s client.

If you want a callback to trigger a calculation, you have two options:

  • store the results of the calculation in a hidden div as mentioned above
  • perform the calculation and then set the output to be a hidden div and just return None or even raise an Exception
  • perform the calculations one-time on app start
  • perform the calcuations on page-load by setting app.layout to a function (as here: Live Updates | Dash for Python Documentation | Plotly)
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