How Can I Efficiently Pass Variables Between Pages in a Flask Application?

Passing Variables Between Flask Pages

In Flask applications, it is often necessary to share data between different pages. This could include passing a user's session information, a form submission, or any other data that needs to be accessible on multiple pages.

One method for passing variables is through the session. The session is a server-side dictionary that stores data for a particular user. It persists between requests, making it a suitable option for passing variables that do not need to be visible to the client.

To use the session, simply set the desired value using the session dictionary. For example:

@app.route('/a')
def a():
    session['my_var'] = 'my_value'
    return redirect(url_for('b'))

On the target page, you can then retrieve the variable from the session using:

@app.route('/b')
def b():
    my_var = session.get('my_var', None)
    return my_var

However, there is a limitation to the size of the session data, typically around 4000 bytes. If you need to pass large amounts of data, it is recommended to use a database or other data storage solution.

Another approach to passing variables is through query parameters. Query parameters are appended to the URL, making them visible to the client. This method is useful for passing small amounts of data that do not need to be stored on the server.

To use query parameters, simply append the variable to the URL using the url_for function:

<a href="{{ url_for('b', my_var='my_value') }}">Send my_value</a>

This will produce the URL:

/b?my_var=my_value

On the target page, you can retrieve the query parameter using the request.args object:

@app.route('/b')
def b():
    my_var = request.args.get('my_var', None)

Both methods for passing variables between Flask pages have their advantages and drawbacks. The session is more secure and persists between requests, while query parameters are more lightweight and allow for data to be visible to the client. Choose the method that best suits your needs.

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