Can I overwrite a static file dynamically using Django? - django

Question

I'm setting up a Heroku site to deploy a Django app for a school project. The problem is with static files using whitenoise in Django.
Quick context: My app consists of a form that takes 4 values that I use for quick math calculation inside a script. The aim of this script is to perform the calculations, draw a plot using matplotlib and save it in the static folder of my django app replacing the old one if it already exists. This static file is used to display in a html page on the site. Locally It works like a charm updating the plot each time I submit a new form. But when I try on Heroku it throws an
[Errno 2] No such file or directory: '/Users/jeff/Desktop/trydjango/src/static/yield_curve.png'
when I submit the form.
Here's the settings.py I have concerning static files:
STATICFILES_DIRS = [os.path.join(BASE_DIR, 'static'),]
STATIC_URL = '/static/'
STATIC_ROOT = os.path.join(BASE_DIR, 'staticfiles')
STATICFILES_STORAGE = 'whitenoise.storage.CompressedManifestStaticFilesStorage'
My directory look like this :
src
|-- /TER
|-- -- /settings.py
|-- /graph
|-- /static/...
|-- /staticfiles/...
|-- /manage.py
I would like for my site to refresh the image each time I submit a form using the new yield_curve.png I saved in the static folder.
If I had to guess, I would say that it has to do with the fact that static files have to be "static" and not change with time.

Answer 1

and save it in the static folder of my django app replacing the old one if it already exists
Yes, this is possible with Django. No, it's not possible on Heroku due to its ephemeral filesystem. You can overwrite your files, but that change will be lost the next time your dyno restarts. This happens frequently (at least once per day).
Heroku officially recommends storing user uploads and static files on a third party service like Amazon S3. Whitenoise disagrees. You can use Whitenoise on Heroku, but you can't (persistently) modify your static files without redeploying.
Note that this is true regardless of the plan your dynos use. Free or enterprise, dyno filesystems are ephemeral.

Answer 2

I believe you cannot save/update files with the free/basic Heroku account. Static files have to remain static and unchanged. I tried to do the same thing years ago with a school project myself. Here is how I pass values to my JS charts to render a figure dynamically so I don't have to use a file:
views.py
def some_function(request):
# Do your calculations on the data here
data = [1,2,3,4] # Let's say this is the results
# Pass data in your context to the template
context = { "my_data" : data }
my_template.html
<!-- Before including your JS file -->
<script> var my_passed_data = {{ my_data|safe }} </script>
<script "include your JS file (test.js in this example) which uses the my_passed_data to make a figure>
test.js
// Whatever library you use to render a chart will be here, I'm using CanvasJS for this example
var chart = new CanvasJS.Chart("chartContainer", {
** all your options, etc. **
data: [
{
** all the other stuff such as type, name, etc. **
dataPoints: [
{ x: 0, y: my_passed_data[0] }, // here is where you access your passed data by index
{ x: 2, y: my_passed_data[1] },
This is how I pass data from my View to Template to JS to render charts using CanvasJS and without needing any files. I'm not sure if this is the best way but it works for me, hope this helps and good luck with your school project!