FastAPI - Dataframe updated change lost between route

Question

I'm trying to make a simple FastAPI api.

Let's suppose these routes:

The POST Route

@api.post('/user', name='Get list of users')
def get_user(user: User):

    """Returns a list of users
    """

    res = get_user(data, user.name, user.nick, user.mail)

    return {
        'username': user.name,
        'nickname': user.nick,
        'user': user.mail
        'USERS': res
    }

And the PUT Route

@api.put('/add_user', name='Create a new user')
def add_question(new_user: New_User):
        u = {
            'name': new_user.name,
            'nick': new_user.nick,
            'mail': new_user.mail
        }

        global data

        data = data.append(u, ignore_index=True)

        return parse_csv(data)

The 'data' is a pandas dataframe (from a csv). And the 'parse_csv()' function is an helper function which transform my dataframe to json.

Now, my issue is when I add a user through the PUT route, the user is correctly added (I can see it via the return), and if I made a request with my GET route, the dataframe is not updated.

How can I update my dataframe without loose any change between route usage ?

I precise it's an API for test purpose, so that's the reason why i'm using a csv file.

Thank's a lot.