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I would like to set a global variable and use it as a trigger of various functions. Each user has a separate global variable. This is used to keep track of previous message data and proceed a conversation. The problem is that how can I manage a separate global variable to each user? The app is running once I deployed it in the server. When I attempt to change the global variable, this variable applies to every user, not just that single user who triggered its the change.

I'm using python flask without a DB.

Thank you.

Answers

You don't need a global variable for this and truly speaking you won't need them anytime in the future as it is a bad practice to use global variables. Refer to this link for details, why are global variables evil?

Now coming to your problem, you probably need g module of flask which creates a context which persist over multiple requests from the same user. You can do something like this:

from flask import g

...

def get_messages():
    messages = getattr(g, '_messages', None)
    if messages is None:
        g._messages = []  # to store messages you may use a dictionary

    return g._messages

def add_message(message):
    messages = get_messages()
    messages.append(message)
    setattr(g, '_messages', messages)
    return messages

And remember for each user a different thread of the app is created so the neither the variables are shared nor their values. So for each user there will be a different g but it will persist over multiple requests from the same user. Hope it helps!

Edit: g object is a proxy to application context and doesn't persists across multiple requests. It persists for a single requests across the application.