Unfortunately, global variables aren’t safe to use in Dash for a couple of reasons:
1 - Mutations of global variable will persist across sessions causing one user’s session to influence the next user’s session.
2 - The global variable mutations will not persist across python processes. It is necessary to run Dash across multiple processes (with something like gunicorn) in production environments so that Dash can handle more than 1 request at once.
However, there are a couple of workarounds for this workflow. See Working on large datasets -- comparison with shiny for more details