Python Flask session for multiple instances
Default Flask sessions are stored client-side (in the browser) as a cookie and cryptographically signed to prevent tampering. Every request to your Flask application is accompanied by this cookie. Therefore, if all running containers have the same app (at least the same secret key used for signing), then they should all have access to the same session data.
Note:

This is cryptographically signed, but it is not encrypted, so don't store sensitive information in the session
Flask-Session can be installed for server-side session support
How can I make a flask session work across multiple instances of containers using Kubernetes? Or does Kubernetes always maintain the same container for a given session?