A way to detect session expiration in flask? - flask

Question

In Flask I want to delete some files when the session times out. I don't have user-login - anyone can upload images and do stuff with them. I want the images to disappear when the session does. But how do I detect when that is?
I believe it will not make sense to store the images in RAM, that's why I want to save them on the server and remove them again.
edit: code for view function and html for rendering PDF with images that are to be deleted when session times out.
views.py:
from flask import (render_template, request)
from flask_weasyprint import HTML, render_pdf
from .forms import toPDF
from .helpers import (custom_images_in_collection)
#user_blueprint.route("/collection", methods=["GET", "POST"])
def collection():
form = toPDF()
# 'collection' is a list of preexisting/permanent image objects selected by user and stored in session
# custom_images is a list of extra image objects created by user on the fly. They're stored
# and retreived from the database from their own table of temporary images with a time field.
# I want the custom images to disappear when the session expires
custom_images = custom_images_in_collection(collection)
collection.append(custom_images)
if request.method == "POST":
if collection:
if form.validate_on_submit():
template = render_template("mypdf.html", collection=collection)
html = HTML(string=template)
# This bit of CSS is dynamically generated, the rest is hard coded in the template
return render_pdf(html)
return render_template("collection.html", collection=collection, form=form)
mypdf.html:
{% if collection|length %}
<div class="innerbox">
{% for word in collection %}
<div class="imgcard">
<div class="imgcontainer">
<img class="card-img-top" src="{{ url_for('static', filename='images/' + word.image.name)}}"
alt="image of '{{ word.word }}'">
</div>
<div class="card-body">
<h2 class="card-title">{{ word.word }}</h2>
</div>
</div>
</div>
{% endfor %}
{% else %}
<div class="col-4">
<p>You don't have a collection yet!</p>
</div>
{% endif %}